DOC/TST: index followup

[jreback]

TST: argmin/max impl, tests, and doc-strings
DOC: release notes corrections
API: raise on __nonzero__ for Indexes, closes #7897

partial close of #7904

[jreback]

@jorisvandenbossche this fixes the missing properties from api docs and such, however

some properties, that are cached_read_only are not having the doc-strings picked up

e.g. Index.is_monotonic,is_unique....

do you know what the problem is here?

[jorisvandenbossche]

Nope, I don't really know what the exact cause is, it's a long standing issue, see #6100 (it happens in two cases: when an attribute is initialized as class attribute to None (eg Index.name), or when cache_readonly is used for properties (eg Index.is_unique))

[jreback]

@jorisvandenbossche hmm, ok then