Update groupby.rst
Update groupby.rst

Edit following this remark <https://github.com/pydata/pandas/pull/6304/files#r9564092>
BUG: correctly handle placement of pos/neg inf when dividing by integer 0 (GH6178)
ENH/BUG: allow single versus multi-index joining on inferred level (GH3662)
Merge pull request #6304 from mazieres/patch-1

Add example section to groupby.rst and one example
DOC: minor groupby.rst edits
CLN: clarify code in com.mask_missing
BUG: disallow comparisons in replace_list with malformed types
Merge pull request #6359 from jreback/int_div

BUG: correctly handle placement of pos/neg inf when dividing by integer 0)
DOC: release.rst correction
Merge pull request #6363 from jreback/mi_merge_single

ENH/BUG: allow single versus multi-index joining on inferred level (GH3662)
DOC: add small caveat section in missing_data.rst
Merge pull request #6354 from cpcloud/bool-block-fail-6353

BLD/TST: fix bool block failures when strings are passed to replace list