Elaborate description in release notes

Parent selection is only applicable for merge commits

Change-Id: I0dc3129306fc51a8d561169a8de009cb5565f2fc
diff --git a/ReleaseNotes/ReleaseNotes-2.13.txt b/ReleaseNotes/ReleaseNotes-2.13.txt
index 3df31dd..2bd20ea 100644
--- a/ReleaseNotes/ReleaseNotes-2.13.txt
+++ b/ReleaseNotes/ReleaseNotes-2.13.txt
@@ -141,7 +141,7 @@
 ==== Change Screen
 
 * link:https://bugs.chromium.org/p/gerrit/issues/detail?id=106[Issue 106]:
-Allow to select parent for diff base in change screen.
+Allow to select merge commit's parent for diff base in change screen.
 
 * link:https://bugs.chromium.org/p/gerrit/issues/detail?id=3035[Issue 3035]:
 Allow to remove specific votes from a change, while leaving the reviewer on the