I was a bit surprised to find that domains were almost completely
unmentioned in the main SGML documentation, outside of the reference
pages for CREATE/ALTER/DROP DOMAIN.  In particular, noplace was it
mentioned that we don't support domains over composite, making it
hard to document the planned fix for that.

Hence, add a section about domains to chapter 8 (Data Types).

Also, modernize the type system overview in section 37.2; it had never
heard of range types, and insisted on calling arrays base types, which
seems a bit odd from a user's perspective; furthermore it didn't fit well
with the fact that we now support arrays over types other than base types.
It seems appropriate to use the term "container types" to describe all of
arrays, composites, and ranges, so let's do that.

Also a few other minor improvements, notably improve an example query
in rowtypes.sgml by using a LATERAL function instead of an ad-hoc
OFFSET 0 clause.

In part this is mop-up for commit c12d570f, which missed updating 37.2
to reflect the fact that it added arrays of domains.  We could possibly
back-patch this without that claim, but I don't feel a strong need to.