Improvements to the partitioning documentation. Original patch from Greg

Improvements to the partitioning documentation. Original patch from Greg
Stark, additional fixes and editorialization by Neil Conway. Greg's patch
description:

    Some more doc patches for partitioned tables. In particular replace
    the caveat that INCLUDING CONSTRAINTS doesn't exist and replace it
    with documentation of, well, INCLUDING CONSTRAINTS.

    Also, there was an instance of "LIKE WITH DEFAULTS" which is
    actually spelled "LIKE INCLUDING DEFAULTS".