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.
BUG: Bug in TimeGrouper/resample when presented with a non-monotonic DatetimeIndex would return invalid results. (GH4161)
BUG: Bug in index name propogation in TimeGrouper/resample (GH4161)
API: TimeGrouper has a more compatible API to the rest of the groupers (e.g. ``groups`` was missing) (GH3881)
Merge pull request #6350 from jreback/groupby_sorted

BUG: TimeGrouper sortedness / API fix (GH4161,GH3881)
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