pgsql: 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: /messages/by-id/5BD1284C.1010305@anastigmatix.net
Discussion: /messages/by-id/5C81F8C0.6090901@anastigmatix.net
Discussion: /messages/by-id/CAN-V+g-6JqUQEQZ55Q3toXEN6d5Ez5uvzL4VR+8KtvJKj31taw@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/12d46ac392d00fa0d103f445aa4997f97029e007

Modified Files
--------------
doc/src/sgml/datatype.sgml | 5 +
doc/src/sgml/features.sgml | 334 ++++++++++++++++++++++++++++++++++-
doc/src/sgml/func.sgml | 224 +++++++++++++----------
src/backend/catalog/sql_features.txt | 6 +-
4 files changed, 475 insertions(+), 94 deletions(-)