Document problems with release links in early branches.

doc/src/sgml/release.sgml

-<!-- $PostgreSQL: pgsql/doc/src/sgml/release.sgml,v 1.490 2007/01/05 22:34:35 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/release.sgml,v 1.491 2007/01/06 15:19:45 momjian Exp $ -->
 <!--
 Typical markup:
@@ -27,7 +27,8 @@ wrap long lines
 For new features, add links to the documentation sections.  Use </link>
 so that perl can remove it so HISTORY.html can be created with no
-links to the main documentation.
+links to the main documentation.  This was added only in 8.2, so don't
+do it for earlier branch release files.
 -->