Add compatibility note about grant options on GRANT reference page

Point out in the compatibility section that granting grant options to
PUBLIC is not supported by PostgreSQL.  This is already mentioned
earlier, but since it concerns the information schema, it might be
worth pointing out explicitly as a compatibility issue.

doc/src/sgml/ref/grant.sgml

@@ -630,6 +630,12 @@ GRANT admins TO joe;
     <quote>_SYSTEM</>, the owner cannot revoke these rights.
    </para>
 
+   <para>
+    According to the SQL standard, grant options can be granted to
+    <literal>PUBLIC</literal>; PostgreSQL only supports granting grant options
+    to roles.
+   </para>
+
    <para>
     The SQL standard provides for a <literal>USAGE</literal> privilege
     on other kinds of objects: character sets, collations,