Add mention of NULL for COMMENT.

doc/src/sgml/ref/comment.sgml

 <!--
-$Header: /cvsroot/pgsql/doc/src/sgml/ref/comment.sgml,v 1.2 1999/10/26 03:53:35 momjian Exp $
+$Header: /cvsroot/pgsql/doc/src/sgml/ref/comment.sgml,v 1.3 1999/10/26 16:36:28 momjian Exp $
 Postgres documentation
 -->
 
@@ -102,7 +102,7 @@ COMMENT
    <command>COMMENT</command> adds a comment to an object that can be 
     easily retrieved with <application>psql's</application>
     <emphasize>\dd</emphasize> command.
-    To remove a comment, use <literal>''</literal>.
+    To remove a comment, use <literal>NULL</literal>.
     Comments are automatically dropped when the object is dropped.
   </para>
  </refsect1>