Update groupby.rst
Update groupby.rst

Edit following this remark <https://github.com/pydata/pandas/pull/6304/files#r9564092>
Merge pull request #6304 from mazieres/patch-1

Add example section to groupby.rst and one example
DOC: minor groupby.rst edits
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)
WIP: add multi-multi index support to join