pgsql: Update, add mention of user locking table before MERGE: < so

Log Message:
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Update, add mention of user locking table before MERGE:

< so duplicate checking can be easily performed.

so duplicate checking can be easily performed. It is possible to
do it without a unique index if we require the user to LOCK the table
before the MERGE.

Modified Files:
--------------
pgsql/doc:
TODO (r1.1698 -> r1.1699)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/TODO.diff?r1=1.1698&amp;r2=1.1699)
pgsql/doc/src/FAQ:
TODO.html (r1.205 -> r1.206)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/FAQ/TODO.html.diff?r1=1.205&amp;r2=1.206)