DOC: More 0.22.0 updates
[ci skip]

cc @jorisvandenbossche 

xref https://github.com/pandas-dev/pandas/issues/18985#issuecomment-354525713 and https://github.com/pandas-dev/pandas/pull/18983#issuecomment-354525079
Is there a 0.21.2 to be released? In the doc string for ``.sum`` and ``prod`` it says that these were added in 0.21.2 0.22.0 is the next release

@topper-123 that is on master (should be 0.22.0). This is for the 0.22.0 branch.

@TomAugspurger  I think that's it for 0.22. Going to tag after backporting it.