[Issue: #16483] Adding GroupBy Filter to API docs.

[VincentLa]

• closes DOC: filter method for *GroupBy objects #16483
• tests added / passed
• passes git diff upstream/master --name-only -- '*.py' | flake8 --diff
• whatsnew entry

[TomAugspurger]

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?

[VincentLa]

@TomAugspurger doesn't look like there are any sphinx warnings!

[codecov]

Codecov Report

Merging #16484 into master will not change coverage.
The diff coverage is n/a.

@@           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)	⬆️
#single	40.17% <ø> (ø)	⬆️

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[jreback]

thanks!