API: add 'level' kwarg to 'Index.isin' method

PERF: optimize MultiIndex.isin when level != None

DOC: update doc bits on Index.isin and MultiIndex.isin

DOC: merge isin sections for container & index classes

DOC: make isin a separate section

It's no longer a subsection of boolean indexing.
DOC: small fixes categorical docs
DOC: Fix release note for GH7798
REGR: Regression in multi-index indexing with a non-scalar type object (GH7914)
Merge pull request #7920 from sinhrks/rn7798

DOC: Fix release note for GH7798
Merge pull request #7917 from jorisvandenbossche/docs-categorical-fixes

DOC: small fixes categorical docs
Merge pull request #7921 from jreback/mi_index

REGR: Regression in multi-index indexing with a non-scalar type object (GH7914)
BUG/DOC: Categoricals now raise NotImplementedError when writing to HDFStore with a Fixed type store (GH7918)
         Slicing bug with a single-dtyped category and a possible view
Merge pull request #7892 from immerrr/add-level-kwarg-for-index-isin

API: add 'level' kwarg to 'Index.isin' method
Merge pull request #7924 from jreback/cat_fixes

BUG/DOC: Categorical fixes (GH7918)
BUG: Bug in to_datetime when format='%Y%m%d and coerce=True are specified, where previously an object array was returned (rather than
     a coerced time-series with NaT), (GH7930)
Merge pull request #7931 from jreback/dt_format

BUG: Bug in to_datetime when format='%Y%m%d and coerce=True are specified (GH7930)
DOC: update docs to show construction of periodindex when needing out-of-bounds spans
Merge pull request #7933 from jreback/oob

DOC: update docs to show construction of periodindex when needing out-of bounds spans