Improve CREATE POLICY documentation

Improve CREATE POLICY documentation

Clarify that SELECT policies are now applied when SELECT rights
are required for a given query, even if the query is an UPDATE or
DELETE query.  Pointed out by Noah.

Additionally, note the risk regarding concurrently open transactions
where a relation which controls access to the rows of another relation
are updated and the rows of the primary relation are also being
modified.  Pointed out by Peter Geoghegan.

Back-patch to 9.5.