DOC: Clarify that DataFrame string formatters must return strings

I am not 100% sure that string formatters must really return *unicode* strings
(non-unicode strings seem to work, too), but I am leaving that because it's
what is already documentated.

Fixes #6052.
Update groupby.rst
Update groupby.rst

Edit following this remark <https://github.com/pydata/pandas/pull/6304/files#r9564092>
BUG: properly tokenize & and | and exprs with strings
PERF: speed up slice indexing on Series with string keys

The patch avoids scanning the whole slice in Index.is_all_dates check.
Merge branch 'fix-generic-is-all-dates-performance' of https://github.com/immerrr/pandas into immerrr-fix-generic-is-all-dates-performance

Conflicts:
	doc/source/release.rst
Merge branch 'query-nested-strings' of https://github.com/cpcloud/pandas into cpcloud-query-nested-strings
ENH/VIS add kind='hexbin' to df.plot
Merge pull request #5478 from TomAugspurger/hexbin-plot

VIS/ENH Hexbin plot
Merge pull request #6054 from shoyer/test-DataFrame.to_latex

DOC: Clarify that DataFrame string formatters must return strings
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