pgsql: Better document that SET ROLE does not uset ALTER ROLE SET

Log Message:
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Better document that SET ROLE does not uset ALTER ROLE SET settings;
suggested wording from Josh Berkus.

Modified Files:
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pgsql/doc/src/sgml/ref:
alter_role.sgml (r1.11 -> r1.12)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/alter_role.sgml?r1=1.11&r2=1.12)
set_role.sgml (r1.6 -> r1.7)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/set_role.sgml?r1=1.6&r2=1.7)