pgsql: Fix documentation error: GRANT/REVOKE for roles only accept role

Log Message:
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Fix documentation error: GRANT/REVOKE for roles only accept role names
as grantees, not PUBLIC ... and you can't say GROUP either.  Noted by
Brian Hurt.

Tags:
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REL8_1_STABLE

Modified Files:
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    pgsql/doc/src/sgml:
        user-manag.sgml (r1.33 -> r1.33.2.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/user-manag.sgml.diff?r1=1.33&r2=1.33.2.1)
    pgsql/doc/src/sgml/ref:
        grant.sgml (r1.50 -> r1.50.2.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/grant.sgml.diff?r1=1.50&r2=1.50.2.1)
        revoke.sgml (r1.35 -> r1.35.2.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/revoke.sgml.diff?r1=1.35&r2=1.35.2.1)