Describe changelog quirk around beta release

This helps guide users who are reading changelogs better understand what
changes are a part of a "main" release.

docs/html/news.rst

@@ -2,4 +2,9 @@
 Changelog
 =========
 
+.. attention::
+
+    Major and minor releases of pip also include changes listed within
+    prior beta releases.
+
 .. include:: ../../NEWS.rst