Complete the documentation of the USAGE privilege for foreign servers

The GRANT reference page failed to mention that the USAGE privilege
allows modifying associated user mappings, although this was already
documented on the CREATE/ALTER/DROP USER MAPPING pages.

doc/src/sgml/ref/grant.sgml

@@ -339,8 +339,10 @@ GRANT <replaceable class="PARAMETER">role_name</replaceable> [, ...] TO <replace
        to create new servers using that foreign-data wrapper.
       </para>
       <para>
-       For servers, this privilege enables the grantee to query the
-       options of the server and associated user mappings.
+       For servers, this privilege enables the grantee to create,
+       alter, and drop his own user's user mappings associated with
+       that server.  Also, it enables the grantee to query the options
+       of the server and associated user mappings.
       </para>
      </listitem>
     </varlistentry>