Improve documentation about CREATE TABLE ... LIKE.

The docs failed to explain that LIKE INCLUDING INDEXES would not preserve
the names of indexes and associated constraints.  Also, it wasn't mentioned
that EXCLUDE constraints would be copied by this option.  The latter
oversight seems enough of a documentation bug to justify back-patching.

In passing, do some minor copy-editing in the same area, and add an entry
for LIKE under "Compatibility", since it's not exactly a faithful
implementation of the standard's feature.

Discussion: <20160728151154.AABE64016B@smtp.hushmail.com>