Improve documentation about our XML functionality.

Improve documentation about our XML functionality.

Add a section explaining how our XML features depart from current
versions of the SQL standard.  Update and clarify the descriptions
of some XML functions.

Chapman Flack, reviewed by Ryan Lambert

Discussion: https://postgr.es/m/5BD1284C.1010305@anastigmatix.net
Discussion: https://postgr.es/m/5C81F8C0.6090901@anastigmatix.net
Discussion: https://postgr.es/m/CAN-V+g-6JqUQEQZ55Q3toXEN6d5Ez5uvzL4VR+8KtvJKj31taw@mail.gmail.com