pgsql: PL/pgSQL FOUND Document that GET DIAGNOSTICS is affected by

Log Message:
-----------
PL/pgSQL FOUND

Document that GET DIAGNOSTICS is affected by EXECUTE, while FOUND is
not.

Modified Files:
--------------
pgsql/doc/src/sgml:
plpgsql.sgml (r1.144 -> r1.145)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/plpgsql.sgml?r1=1.144&r2=1.145)