[1.6.x] Fixed #13755 -- Added a tip for caching responses that vary on AJAX.

Thanks mila for the suggestion.

Backport of db9a1a0c47 from master

docs/ref/request-response.txt

@@ -310,6 +310,13 @@ Methods
    If you write your own XMLHttpRequest call (on the browser side), you'll
    have to set this header manually if you want ``is_ajax()`` to work.
 
+    If a response varies on whether or not it's requested via AJAX and you are
+    using some form of caching like Django's :mod:`cache middleware
+    <django.middleware.cache>`, you should decorate the view with
+    :func:`vary_on_headers('HTTP_X_REQUESTED_WITH')
+    <django.views.decorators.vary.vary_on_headers>` so that the responses are
+    properly cached.
+
 .. method:: HttpRequest.read(size=None)
 .. method:: HttpRequest.readline()
 .. method:: HttpRequest.readlines()