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.

doc/src/sgml/ref/create_role.sgml

@@ -170,7 +170,7 @@ CREATE ROLE <replaceable class="PARAMETER">name</replaceable> [ [ WITH ] <replac
         If not specified,
         <literal>NOLOGIN</literal> is the default, except when
         <command>CREATE ROLE</> is invoked through its alternative spelling
-        <command>CREATE USER</>.
+        <xref linkend="sql-createuser">.
        </para>
       </listitem>
      </varlistentry>