The two "XML Fixes" patches still in need of review

Alvaro has committed two of the patches in this CF entry[1], but the
remaining two have yet to attract review.

This message contains only those two, just as before[2] except rebased
over Alvaro's commits of the others.

<confession>
 There are two new changes. The <ulink>s added in the -docfix patch now
 have link text, as allowed according to b060e6c, and the -content patch
 now updates the definition of the 'content' type in datatypes.sgml,
 which I had overlooked before.
</confession>

xml-functions-type-docfix-3.patch adjusts the documentation of the XML type
and related functions to present some behavior and limitations more clearly.

xml-content-2006-2.patch changes the behavior of xmlparse and the
text-to-xml cast to allow any XML 'document' (including one with a DTD)
to be parsed as 'content', where the former behavior was to fail in that
case. This is the same as changing the definition of XML 'content' from
that of SQL:2003 to that of SQL:2006 and later. The later definition is
preferable, because it eliminates a case that can fail in, e.g., pg_restore
(which problem has been reported in the field).

The patches apply in that order (because the -docfix one adds language
describing the current 'content' behavior, then the -content one changes
the behavior, and the language to match it).

Regards,
-Chap

[1] https://commitfest.postgresql.org/22/1872/
[2]
https://www.postgresql.org/message-id/3e8eab9e-7289-6c23-5e2c-153cccea2257@anastigmatix.net