CLN: Change assert_(a [not] in b) to specialized forms

Work on #6175. Changes instances of assert_(a [not] in b) to specialized assert[Not]In(a, b).
DOC: small doc fixes
Merge pull request #6442 from jorisvandenbossche/doc-fixes

DOC: small doc fixes
Merge pull request #6395 from bwignall/assert_6175_innotin

CLN: Change assert_(a [not] in b) to specialized forms
TST: fix spurious tests for test_groupby (GH6436)
BUG/TST: iloc will now raise IndexError on out-of-bounds list indexers to promotoe
         consistency with python/numpy syntax. The out-of-bounds for slice indexers
         will continue to work (again for consistency) (GH6296 / GH6299)
DOC: fix .tz attribute error in DatetimeIndex when building docs
Merge pull request #6443 from jreback/iloc_bounds2

BUG/TST: iloc will now raise IndexError on out-of-bounds list indexers (GH6296 / GH6299)