pgsql-server: Add discussion and example about predicate locking and why

Log Message:
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Add discussion and example about predicate locking and why "serializable"
mode isn't really serializable.  I had thought this was covered already
in our docs, but I sure can't find it.

Modified Files:
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    pgsql-server/doc/src/sgml:
        mvcc.sgml (r2.43 -> r2.44)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/doc/src/sgml/mvcc.sgml.diff?r1=2.43&r2=2.44)