DOC: Modify IntervalArray/IntervalIndex docstrings to pass validate_docstrings.py
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Didn't materially change any existing content; mostly small modifications and a couple additions.
Hello @jschendel! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/arrays/interval.py`](https://github.com/jschendel/pandas/blob/fe5a3ef3bb7d6be60d256a763b376ccad55aeaac/pandas/core/arrays/interval.py) !

 - There are no PEP8 issues in the file [`pandas/core/indexes/interval.py`](https://github.com/jschendel/pandas/blob/fe5a3ef3bb7d6be60d256a763b376ccad55aeaac/pandas/core/indexes/interval.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23157?src=pr&el=h1) Report
> Merging [#23157](https://codecov.io/gh/pandas-dev/pandas/pull/23157?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/1546c35dc839b3f6c094ad41d14cda24e9e13a0c?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23157/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23157?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23157   +/-   ##
=======================================
  Coverage   92.19%   92.19%           
=======================================
  Files         169      169           
  Lines       50954    50954           
=======================================
  Hits        46975    46975           
  Misses       3979     3979
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.61% <100%> (ø)` | :arrow_up: |
| #single | `42.27% <100%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23157?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/interval.py](https://codecov.io/gh/pandas-dev/pandas/pull/23157/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9pbnRlcnZhbC5weQ==) | `94.63% <ø> (ø)` | :arrow_up: |
| [pandas/core/arrays/interval.py](https://codecov.io/gh/pandas-dev/pandas/pull/23157/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2ludGVydmFsLnB5) | `92.81% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23157?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/23157?src=pr&el=footer). Last update [1546c35...af757a0](https://codecov.io/gh/pandas-dev/pandas/pull/23157?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Added `indexes/interval.py` and `arrays/interval.py` to the `ci/code_checks.sh`.  Had to make it ignore some methods that aren't passing yet, but will get to those shortly in a separate PR.