Make the reference to "CREATE USER" in the CREATE ROLE page a link.

Make the reference to "CREATE USER" in the CREATE ROLE page a link.

This might help to avoid confusion between the CREATE USER command,
and the deprecated CREATEUSER option to CREATE ROLE, as per a recent
complaint from Ron Adams.  At any rate, having a cross-link here
seems like a good idea; two commands that are so similar should
reference each other.