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

  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