[1.7.x] Fixed #22966 -- Clarified which release notes appear for each doc version.

Thanks haimunt at yahoo.com for the suggestion.

Backport of e6b3d6c2 from master

docs/releases/index.txt

@@ -15,6 +15,9 @@ version.
 Final releases
 ==============
 
+Below are release notes through Django |version| and its minor releases. Newer
+versions of the documentation contain the release notes for any later releases.
+
 .. _development_release_notes:
 
 1.7 release