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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