[Issue: #16483] Adding GroupBy Filter to API docs.
 - [ ] closes #16483
 - [ ] tests added / passed
 - [ ] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [ ] whatsnew entry

When the doc build finishes https://travis-ci.org/pandas-dev/pandas/jobs/235778446 can you search through the log for "filter" and see if there are any sphinx warnings? @TomAugspurger doesn't look like there are any sphinx warnings! # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16484?src=pr&el=h1) Report
> Merging [#16484](https://codecov.io/gh/pandas-dev/pandas/pull/16484?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/97ad3fb9c87226ad983267e2891dbbf68432b8ea?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/16484/graphs/tree.svg?token=eZ4WkYLtcO&width=650&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/16484?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #16484   +/-   ##
=======================================
  Coverage    90.4%    90.4%           
=======================================
  Files         161      161           
  Lines       51033    51033           
=======================================
  Hits        46136    46136           
  Misses       4897     4897
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.24% <ø> (ø)` | :arrow_up: |
| #single | `40.17% <ø> (ø)` | :arrow_up: |


------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16484?src=pr&el=footer). Last update [97ad3fb...301f355](https://codecov.io/gh/pandas-dev/pandas/pull/16484?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!