Add documentation to pandas.core.groupby.GroupBy.apply
It took me a few hours to figure out how to edit rows from `groupBy` objects until I scavenged the code.

Hello @Rafalonso! Thanks for updating the PR.



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

##### Comment last updated on February 27, 2018 at 19:52 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19927?src=pr&el=h1) Report
> Merging [#19927](https://codecov.io/gh/pandas-dev/pandas/pull/19927?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f4c9d966fb20d9ca3221d31bd9eb31117065808c?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19927/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/19927?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19927      +/-   ##
==========================================
+ Coverage   91.66%   91.69%   +0.02%     
==========================================
  Files         150      150              
  Lines       48974    48974              
==========================================
+ Hits        44894    44906      +12     
+ Misses       4080     4068      -12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.07% <ø> (+0.02%)` | :arrow_up: |
| #single | `41.85% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19927?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/19927/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | `92.31% <ø> (ø)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19927/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `66.95% <0%> (+1.73%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19927?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/19927?src=pr&el=footer). Last update [f4c9d96...17d3037](https://codecov.io/gh/pandas-dev/pandas/pull/19927?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @pandas-dev/pandas-core needs a rebase @jreback : Given @TomAugspurger comment [above](https://github.com/pandas-dev/pandas/pull/19927#discussion_r171063351), I wonder if it might be best to close.  This PR has been dormant for a long time, and I'm not seeing the immediate benefit of this addition unfortunately. After consideration I agree that this PR doesn't provide much value. Thanks for the comments.