[COMMITTERS] pgsql: Documentation improvements for pg_locks with respect to SSI.

Documentation improvements for pg_locks with respect to SSI. Explain that querying pg_locks does not simultaneously lock both the normal lock manager and the predicate lock manager. Per discussion with Kevin Grittner. Branch -- master Details ---

[COMMITTERS] pgsql: Documentation improvements for pg_locks with respect to SSI.

Documentation improvements for pg_locks with respect to SSI. Explain that querying pg_locks does not simultaneously lock both the normal lock manager and the predicate lock manager. Per discussion with Kevin Grittner. Branch -- REL9_1_STABLE Details ---