Update Changelog entry for 5.2.4.1 to warn of breaking changes

It seems that the changes in 5.2.4.1 have led to a number of discussions on commits, issues and pull requests. Clarifying the Changelog entry seems like the most direct way to help people apply the security patch smoothly.

actionpack/CHANGELOG.md

@@ -26,6 +26,11 @@
 
     The `ActionDispatch::Session::MemcacheStore` is still vulnerable given it requires the
     gem dalli to be updated as well.
+
+    _Breaking changes:_
+    *   `session.id` now returns an instance of `Rack::Session::SessionId` and not a String (use `session.id.public_id` to restore the old behaviour, see #38063)
+    *   Accessing the session id using `session[:session_id]`/`session['session_id']` no longer works with
+        ruby 2.2 (see https://github.com/rails/rails/commit/2a52a38cb51b65d71cf91fc960777213cf96f962#commitcomment-37929811)
 
     CVE-2019-16782.