Doc: sync CREATE GROUP syntax synopsis with CREATE ROLE.

 Doc: sync CREATE GROUP syntax synopsis with CREATE ROLE.
 
 CREATE GROUP is an exact alias for CREATE ROLE, and CREATE USER is
 almost an exact alias, as can easily be confirmed by checking the
 code.  So the man page syntax descriptions ought to match up.  The
 last few additions of role options seem to have forgotten to update
 create_group.sgml, though.  Fix that, and add a naggy reminder to
 create_role.sgml in hopes of not forgetting again.
 
 Discussion: https://postgr.es/m/158647836143.655.9853963229391401576@wrigleys.postgresql.org