pgsql: Make it more clear that you need to release savepoint with

Log Message:
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Make it more clear that you need to release savepoint with
RELEASE SAVEPOINT to make an older savepoint with the same name
accessible. It's also possible to implicitly release the savepoint by
rolling back to an earlier savepoint, but mentioning that too would make
the note just more verbose and confusing.

Modified Files:
--------------
    pgsql/doc/src/sgml/ref:
        savepoint.sgml (r1.7 -> r1.8)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/savepoint.sgml?r1=1.7&r2=1.8)