sw btlr writing mode: add odf spec proposal URL

Change-Id: I7848677cf4c8246fc7a1833bec809945fb541b29
Reviewed-on: https://gerrit.libreoffice.org/67814Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins

schema/libreoffice/OpenDocument-schema-v1.3+libreoffice.rng

@@ -2323,7 +2323,7 @@ xmlns:loext="urn:org:documentfoundation:names:experimental:office:xmlns:loext:1.
     </rng:optional>
   </rng:define>
 
-  <!-- TODO no proposal -->
+  <!-- https://issues.oasis-open.org/browse/OFFICE-4030 -->
   <rng:define name="common-writing-mode-attlist" combine="interleave">
     <rng:optional>
       <rng:attribute name="loext:writing-mode">