pgsql: Improve description of \du and \dg, per suggestion from Harald

Log Message:
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Improve description of \du and \dg, per suggestion from
Harald Armin Massa.

Modified Files:
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pgsql/src/bin/psql:
help.c (r1.123 -> r1.124)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/help.c?r1=1.123&r2=1.124)