Doc Fixes
Closes <#21308> 
Note : Defined Levels section was added in the "Notes" section opposed to "See Also".
Description of "See Also" section [here](https://numpydoc.readthedocs.io/en/latest/format.html), suggests it should really link to other functions etc. 


Hello @alimcmaster1! Thanks for updating the PR.



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

##### Comment last updated on June 11, 2018 at 22:35 Hours UTC @jschendel mind reviewing? This should make this functionality abit clearer.
Thanks,
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21415?src=pr&el=h1) Report
> Merging [#21415](https://codecov.io/gh/pandas-dev/pandas/pull/21415?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/415012f4f38ca0cf41717c51e49bd2349cba09a8?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/21415/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/21415?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21415   +/-   ##
=======================================
  Coverage   91.89%   91.89%           
=======================================
  Files         153      153           
  Lines       49596    49596           
=======================================
  Hits        45576    45576           
  Misses       4020     4020
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.29% <ø> (ø)` | :arrow_up: |
| #single | `41.86% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21415?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexing.py](https://codecov.io/gh/pandas-dev/pandas/pull/21415/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhpbmcucHk=) | `93.55% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21415?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/21415?src=pr&el=footer). Last update [415012f...11fae9b](https://codecov.io/gh/pandas-dev/pandas/pull/21415?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks @gfyoung, added further commit to get rid of my pep8 error causing appveyor CI to fail. Thanks, LGTM pending @TomAugspurger's comment. Thanks both, updated as per @TomAugspurger comment. Also spotted a few other places in the codebase were we could use a Spinx ref. Will raise a separate PR. thanks @alimcmaster1 happy to take more PRs on other patches!