DOC: Cleaned references to pandas <v0.12 in docs
There is a lot of references to in the docs to when exactly some change occured.  For newer changes this is great, but there comes a time when such references only disturb the reader rather than help him, as the versions referenced become so old, that they become noise rather than help.

I've cleaned up references up to and including v0.11. 

IMO I could have gone higher (v.015?), but can do that in some later round.

Some issues I would be glad for input on:
* In ``gotschas.rst``, there is a sentence "As of pandas 0.11, pandas is not 100% thread safe." I haven't altered this, but I presume this still is correct in the newest version of pandas? Then IMO it should be changed to reference a newer version or simply to "pandas is currently not 100% thread safe."
* In ``io.rst`` there is a sentence "0.10.1 of ``HDFStore`` can read tables created in a prior version of pandas, ...". I'm not even sure the "0.10.1" references the version of pandas or a HDF library and I left it alone. The paragraph also discusses backwards compatability, which makes it somewhat relevant to keep around, even if it's an old change.
@topper-123 : I might consider even going to `0.17` in fact because that's > three major releases behind the current one (`0.21`), making super old.

I would leave the `0.11.0` sentence as is in the `gotchas`, and for `io.rst`, given its proximity to `HDFStore` and `pandas`, semantically and grammatically, it is referring to `HDFStore`. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17375?src=pr&el=h1) Report
> Merging [#17375](https://codecov.io/gh/pandas-dev/pandas/pull/17375?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0d676a3ccf1d7aa986416a7488b941496f936d98?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17375/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/17375?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17375      +/-   ##
==========================================
- Coverage   91.01%   90.99%   -0.03%     
==========================================
  Files         163      163              
  Lines       49567    49567              
==========================================
- Hits        45113    45103      -10     
- Misses       4454     4464      +10
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.77% <ø> (-0.01%)` | :arrow_down: |
| #single | `40.25% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17375?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17375?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17375?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.72% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/17375?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.23% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17375?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17375?src=pr&el=footer). Last update [0d676a3...d566383](https://codecov.io/gh/pandas-dev/pandas/pull/17375?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17375?src=pr&el=h1) Report
> Merging [#17375](https://codecov.io/gh/pandas-dev/pandas/pull/17375?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0d676a3ccf1d7aa986416a7488b941496f936d98?src=pr&el=desc) will **increase** coverage by `0.12%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17375/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/17375?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17375      +/-   ##
==========================================
+ Coverage   91.01%   91.14%   +0.12%     
==========================================
  Files         163      163              
  Lines       49567    49581      +14     
==========================================
+ Hits        45113    45190      +77     
+ Misses       4454     4391      -63
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.92% <ø> (+0.15%)` | :arrow_up: |
| #single | `40.25% <ø> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17375?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17375?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17375?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.72% <0%> (-0.1%)` | :arrow_down: |
| [pandas/io/pytables.py](https://codecov.io/gh/pandas-dev/pandas/pull/17375?src=pr&el=tree#diff-cGFuZGFzL2lvL3B5dGFibGVzLnB5) | `92.79% <0%> (-0.09%)` | :arrow_down: |
| [pandas/core/indexes/multi.py](https://codecov.io/gh/pandas-dev/pandas/pull/17375?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9tdWx0aS5weQ==) | `96.9% <0%> (-0.03%)` | :arrow_down: |
| [pandas/io/formats/excel.py](https://codecov.io/gh/pandas-dev/pandas/pull/17375?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvZXhjZWwucHk=) | `96.65% <0%> (ø)` | :arrow_up: |
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/17375?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | `92.21% <0%> (ø)` | :arrow_up: |
| [pandas/core/indexes/api.py](https://codecov.io/gh/pandas-dev/pandas/pull/17375?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9hcGkucHk=) | `98.78% <0%> (ø)` | :arrow_up: |
| [pandas/io/formats/printing.py](https://codecov.io/gh/pandas-dev/pandas/pull/17375?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvcHJpbnRpbmcucHk=) | `89.38% <0%> (ø)` | :arrow_up: |
| [pandas/io/formats/css.py](https://codecov.io/gh/pandas-dev/pandas/pull/17375?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvY3NzLnB5) | `100% <0%> (ø)` | :arrow_up: |
| [pandas/io/parsers.py](https://codecov.io/gh/pandas-dev/pandas/pull/17375?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnNlcnMucHk=) | `95.46% <0%> (ø)` | :arrow_up: |
| ... and [8 more](https://codecov.io/gh/pandas-dev/pandas/pull/17375?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17375?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17375?src=pr&el=footer). Last update [0d676a3...7d18e06](https://codecov.io/gh/pandas-dev/pandas/pull/17375?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 I like this. I think the cutoff you've chosen here is a good one for now. v0.12 marks a slow-down in the development pace of pandas (just look at the pace of release tags).

0.17.0 may be >3 versions old, but it's also less than two years old. In general I would go by time instead of number of major versions.

> In gotschas.rst, there is a sentence "As of pandas 0.11, pandas is not 100% thread safe." I haven't altered this, but I presume this still is correct in the newest version of pandas? Then IMO it should be changed to reference a newer version or simply to "pandas is currently not 100% thread safe."

I don't understand what this was originally intended to convey. There are at least two types of thread-safety:
1. Thread-safe libraries don't crash when run from multiple threads.
2. Thread-safe data structures have locks to ensure atomic operations on mutable data structures.

Like most Python code, pandas falls in the first category, not the second. This is tested routinely by dask. But not even built-in data structures are thread-safe in the second sense in Python.

> In io.rst there is a sentence "0.10.1 of HDFStore can read tables created in a prior version of pandas, ...". I'm not even sure the "0.10.1" references the version of pandas or a HDF library and I left it alone. The paragraph also discusses backwards compatability, which makes it somewhat relevant to keep around, even if it's an old change.

I'm pretty sure 0.10.1 references the pandas version. HDF5 is on version 1.8.17.

I don't think it's important to mention the version here at this point, since 0.10.1 is quite old. >> In io.rst there is a sentence "0.10.1 of HDFStore can read tables created in a prior version of pandas, ...". I'm not even sure the "0.10.1" references the version of pandas or a HDF library and I left it alone. The paragraph also discusses backwards compatability, which makes it somewhat relevant to keep around, even if it's an old change.

> I'm pretty sure 0.10.1 references the pandas version. HDF5 is on version 1.8.17.

this has nothing to do with the HDF5 standard. This is a reference to the pandas version itself. in any event it can be removed.

 Adjusted according to comments.

Wrt. backwards compatabilty of ``HDFStore`` and pandas <0.10.1, I propose the whole paragraph removed. If you're having data as old as pandas 0.10.1, you should be responisible to look through the release notes to find the relevant change. The removal of the note about backwards compatibilit has been moved to #17404, as those two things are connected through the file ``legacy_0.10.h5``. 

IMO this should be ready now to commit.

I will look into cleaning references upto v0.14 or v0.15 as the next step. Ping, corrected. @topper-123 Thanks a lot, this was a good idea!