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.

This is a backpatch for branches 10 and 11, taken from Tom's commit
12d46ac392d0 for 12, then edited to correctly describe behaviors that
are fixed in 12 but still broken in 10 and 11.

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