pgsql: Improve GRANT reference page's description of object ownership

Log Message:
-----------
Improve GRANT reference page's description of object ownership
privileges by mentioning the possibility of granting membership in
the owning role.

Modified Files:
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pgsql/doc/src/sgml/ref:
grant.sgml (r1.74 -> r1.75)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/grant.sgml?r1=1.74&r2=1.75)