Doc: Further improvements for IntervalIndex and Interval
- [x ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

This follows #17706. Further changes are:

* Taken text from the whatsnew for v0.20 and use it in advanced.rst to give more explations to ``IntervalIndex`` and ``Interval``.
* in interval.pyx, the doc string "Attributes" didn't parse properly, because sphinx doesn't like having "left" and "right" on the same line. They're now on separate lines and the section renamed to 'Parameters'. See current here and compare to source: http://pandas-docs.github.io/pandas-docs-travis/generated/pandas.Interval.html#pandas.Interval.
* some examples are added to ``Interval``
* the warning text on ``IntervalIndex`` is now parsed properly.
* The "See Also" had become quite large, felt it becomes more reader-friendly to combine the ``from_*`` metods into one line.
* added ``cut`` and ``qcut`` to 'See also'

This will conclude my contributions to ``IntervalIndex``.
Hello @topper-123! Thanks for updating the PR.



Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on September 29, 2017 at 12:08 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17714?src=pr&el=h1) Report
> Merging [#17714](https://codecov.io/gh/pandas-dev/pandas/pull/17714?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/ad7d051bdfdbadc4221307d691fd55412d9d7ae8?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17714/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/17714?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17714      +/-   ##
==========================================
- Coverage   91.27%   91.25%   -0.02%     
==========================================
  Files         163      163              
  Lines       49765    49783      +18     
==========================================
+ Hits        45421    45430       +9     
- Misses       4344     4353       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.05% <ø> (ø)` | :arrow_up: |
| #single | `40.34% <ø> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17714?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/interval.py](https://codecov.io/gh/pandas-dev/pandas/pull/17714?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9pbnRlcnZhbC5weQ==) | `92.85% <ø> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17714?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17714?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.73% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/tools/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/17714?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvdG9vbHMvZGF0ZXRpbWVzLnB5) | `83.79% <0%> (+0.82%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17714?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/17714?src=pr&el=footer). Last update [ad7d051...0e27cd4](https://codecov.io/gh/pandas-dev/pandas/pull/17714?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 lgtm. ping on green. ping @jreback at green (I assume the failure in CircleCI has nothin to do with my PR). I've been seeing this compilation error fairly regularly:

    pandas/_libs/reshape.pyx:11:0: 'numpy/float16_t.pxd' not found

I'll open a separate issue to track it.