docs: update major release notes item checklist

Author: bmomjian

src/tools/RELEASE_CHANGES

@@ -36,7 +36,7 @@ For Major Releases
 		o  new features and options
 		o  major performance improvements
 		o  bug fixes for serious or common bugs
-		o  incompatibilities
+		o  incompatibilities and significant user-visible changes
 		o  major source code changes
 	o check completion of items that have been marked as completed at
 	  http://wiki.postgresql.org/wiki/Todo