Fixed #22827 -- clarified what each cache middleware does.

Thanks Keryn Knight for the report.

docs/topics/cache.txt

@@ -465,14 +465,14 @@ Then, add the following required settings to your Django settings file:
   the site, or some other string that is unique to this Django instance, to
   prevent key collisions. Use an empty string if you don't care.
 
-The cache middleware caches GET and HEAD responses with status 200, where the request
-and response headers allow. Responses to requests for the same URL with different
-query parameters are considered to be unique pages and are cached separately.
-The cache middleware expects that a HEAD request is answered with the same
-response headers as the corresponding GET request; in which case it can return
-a cached GET response for HEAD request.
-
-Additionally, the cache middleware automatically sets a few headers in each
+``FetchFromCacheMiddleware`` caches GET and HEAD responses with status 200,
+where the request and response headers allow. Responses to requests for the same
+URL with different query parameters are considered to be unique pages and are
+cached separately. This middleware expects that a HEAD request is answered with
+the same response headers as the corresponding GET request; in which case it can
+return a cached GET response for HEAD request.
+
+Additionally, ``UpdateCacheMiddleware`` automatically sets a few headers in each
 :class:`~django.http.HttpResponse`:
 
 * Sets the ``Last-Modified`` header to the current date/time when a fresh