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
CLN: clarify code in com.mask_missing
BUG: disallow comparisons in replace_list with malformed types