Documenting MERGE INTO ONLY ...

While testing MERGE, I noticed that it supports inheritance
hierarchies and the ONLY keyword, but that isn't documented. Attached
is a patch to merge.sgml, borrowing text from update.sgml and
delete.sgml.

I note that there are also a couple of places early in the manual
(advanced.sgml and ddl.sgml) that also discuss inheritance, citing
SELECT, UPDATE and DELETE as examples of (already-discussed) commands
that support ONLY. However, since MERGE isn't mentioned until much
later in the manual, it's probably best to leave those as-is. They
don't claim to be complete lists of commands supporting ONLY, and it
would be a pain to make them that.

Regards,
Dean