DOC: Capitalize docstring summaries
- [x] closes #23881
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] Solves most of the issues raised by the `validate_docstrings.py` script. 

The few issues that still show up on running the script can be attributed to- 
- Docstrings non-existent viz. `tseries.offsets.Tick`, `tseries.offsets.Day` etc. (working on this for a separate PR)
- Auto-generated Doctrings viz. `Series.argmin`, `Timestamp.strptime` etc.
- Docstrings starting with quotes viz. 'Normalizes', 'Unpivots' which were (presumably) added in by the original authors by for a reason.

Hello @varadgunjal! Thanks for updating the PR.

 - There are no PEP8 issues in the file [`pandas/core/frame.py`](https://github.com/varadgunjal/pandas/blob/d6a2d5bc016f52cbbb6370f555f8541adc5eff86/pandas/core/frame.py) !

 - There are no PEP8 issues in the file [`pandas/core/groupby/base.py`](https://github.com/varadgunjal/pandas/blob/d6a2d5bc016f52cbbb6370f555f8541adc5eff86/pandas/core/groupby/base.py) !

 - There are no PEP8 issues in the file [`pandas/core/groupby/groupby.py`](https://github.com/varadgunjal/pandas/blob/d6a2d5bc016f52cbbb6370f555f8541adc5eff86/pandas/core/groupby/groupby.py) !

 - There are no PEP8 issues in the file [`pandas/core/indexes/base.py`](https://github.com/varadgunjal/pandas/blob/d6a2d5bc016f52cbbb6370f555f8541adc5eff86/pandas/core/indexes/base.py) !

 - There are no PEP8 issues in the file [`pandas/core/indexes/datetimelike.py`](https://github.com/varadgunjal/pandas/blob/d6a2d5bc016f52cbbb6370f555f8541adc5eff86/pandas/core/indexes/datetimelike.py) !

 - There are no PEP8 issues in the file [`pandas/core/indexes/multi.py`](https://github.com/varadgunjal/pandas/blob/d6a2d5bc016f52cbbb6370f555f8541adc5eff86/pandas/core/indexes/multi.py) !

 - There are no PEP8 issues in the file [`pandas/core/indexes/range.py`](https://github.com/varadgunjal/pandas/blob/d6a2d5bc016f52cbbb6370f555f8541adc5eff86/pandas/core/indexes/range.py) !

 - There are no PEP8 issues in the file [`pandas/core/panel.py`](https://github.com/varadgunjal/pandas/blob/d6a2d5bc016f52cbbb6370f555f8541adc5eff86/pandas/core/panel.py) !

 - There are no PEP8 issues in the file [`pandas/core/resample.py`](https://github.com/varadgunjal/pandas/blob/d6a2d5bc016f52cbbb6370f555f8541adc5eff86/pandas/core/resample.py) !

 - There are no PEP8 issues in the file [`pandas/core/window.py`](https://github.com/varadgunjal/pandas/blob/d6a2d5bc016f52cbbb6370f555f8541adc5eff86/pandas/core/window.py) !

 - There are no PEP8 issues in the file [`pandas/io/formats/style.py`](https://github.com/varadgunjal/pandas/blob/d6a2d5bc016f52cbbb6370f555f8541adc5eff86/pandas/io/formats/style.py) !

 - There are no PEP8 issues in the file [`pandas/io/json/normalize.py`](https://github.com/varadgunjal/pandas/blob/d6a2d5bc016f52cbbb6370f555f8541adc5eff86/pandas/io/json/normalize.py) !

 - There are no PEP8 issues in the file [`pandas/io/pytables.py`](https://github.com/varadgunjal/pandas/blob/d6a2d5bc016f52cbbb6370f555f8541adc5eff86/pandas/io/pytables.py) !



##### Comment last updated on November 24, 2018 at 21:44 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23886?src=pr&el=h1) Report
> Merging [#23886](https://codecov.io/gh/pandas-dev/pandas/pull/23886?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/90961f2acad5c2db6aa957e2a79df8668f5b02d1?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23886/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23886?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23886   +/-   ##
=======================================
  Coverage   92.31%   92.31%           
=======================================
  Files         161      161           
  Lines       51483    51483           
=======================================
  Hits        47525    47525           
  Misses       3958     3958
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.7% <ø> (ø)` | :arrow_up: |
| #single | `42.43% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23886?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/json/normalize.py](https://codecov.io/gh/pandas-dev/pandas/pull/23886/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2pzb24vbm9ybWFsaXplLnB5) | `96.87% <ø> (ø)` | :arrow_up: |
| [pandas/io/formats/style.py](https://codecov.io/gh/pandas-dev/pandas/pull/23886/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvc3R5bGUucHk=) | `96.69% <ø> (ø)` | :arrow_up: |
| [pandas/core/indexes/multi.py](https://codecov.io/gh/pandas-dev/pandas/pull/23886/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9tdWx0aS5weQ==) | `95.49% <ø> (ø)` | :arrow_up: |
| [pandas/core/groupby/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/23886/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9iYXNlLnB5) | `91.83% <ø> (ø)` | :arrow_up: |
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/23886/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `97.74% <ø> (ø)` | :arrow_up: |
| [pandas/core/panel.py](https://codecov.io/gh/pandas-dev/pandas/pull/23886/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcGFuZWwucHk=) | `97.92% <ø> (ø)` | :arrow_up: |
| [pandas/core/arrays/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/23886/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3RpbWVkZWx0YXMucHk=) | `96.44% <ø> (ø)` | :arrow_up: |
| [pandas/core/resample.py](https://codecov.io/gh/pandas-dev/pandas/pull/23886/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzYW1wbGUucHk=) | `96.99% <ø> (ø)` | :arrow_up: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/23886/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.02% <ø> (ø)` | :arrow_up: |
| [pandas/core/window.py](https://codecov.io/gh/pandas-dev/pandas/pull/23886/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvd2luZG93LnB5) | `96.4% <ø> (ø)` | :arrow_up: |
| ... and [7 more](https://codecov.io/gh/pandas-dev/pandas/pull/23886/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23886?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/23886?src=pr&el=footer). Last update [90961f2...b34280b](https://codecov.io/gh/pandas-dev/pandas/pull/23886?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @datapythonista I've been diving into the code further to see where I can contribute and have noticed that a lot of these issues are repeated in other files that do not show up when the validate script runs. The changes will affect **a lot** of files - including editing/updating some docstrings. Shall go ahead and make the changes there and submit PRs module-by-module? Or should I just stick to the files that the script found for this issue? I think the script should like all the docstrings that do not start with a capital letter in the public API. I'd leave just those in this PR. Fixing the others would be nice, but is not as important, as the docstrings are not in the pandas documentation web. If you take care of them, do it in chunks, huge pull requests make reviews complicated and not very efficient. But there should be issues labeled as "good first issue" that are probably more important at this point. Sorry that the PR has become quite big. I've stuck to fixing issues in the files reported by the `validate_docstrings` script. As discussed previously, I will push corresponding changes to docstrings in all the various other files if and when I push changes to the code therein in the future. can you merge master Thanks @varadgunjal for all the work with this, and thanks @gfyoung for the review.