Fixed #31210 -- Doc'd how to reproduce HttpRequest.is_ajax() in release notes.

docs/releases/3.1.txt

@@ -482,6 +482,10 @@ Miscellaneous
   the new :meth:`.HttpRequest.accepts` method if your code depends on the
   client ``Accept`` HTTP header.
 
+  If you are writing your own AJAX detection method, ``request.is_ajax()`` can
+  be reproduced exactly as
+  ``request.headers.get('x-requested-with') == 'XMLHttpRequest'``.
+
 * The encoding format of cookies values used by
   :class:`~django.contrib.messages.storage.cookie.CookieStorage` is different
   from the format generated by older versions of Django. Support for the old