Per-site cache docs vague about which middleware sets headers

[Keryn Knight <django@…>]

The ​docs currently say:

Additionally, the cache middleware automatically sets a few headers in each HttpResponse:

Sets the Last-Modified header to the current date/time when a fresh (not cached) version of the page is requested.
Sets the Expires header to the current date/time plus the defined CACHE_MIDDLEWARE_SECONDS.
Sets the Cache-Control header to give a max age for the page – again, from the CACHE_MIDDLEWARE_SECONDS setting.

It's no longer clear which middleware does that, because these days what was once CacheMiddleware is now recommended as two separate components, UpdateCacheMiddleware and FetchFromCacheMiddleware. It is the ​UpdateCacheMiddleware that is responsible for modifying the Response's headers.

And, if being really picky:

The cache middleware caches GET and HEAD responses with status 200, where the request and response headers allow

This is dictated by the ​FetchFromCacheMiddleware at the beginning of the request

[mardini]

PR: ​https://github.com/django/django/pull/2955
Thanks.

[Tim Graham <timograham@…>]

In f9b7a0383c6f0fbadb83306db667675d9f9dd2b4:

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

Thanks Keryn Knight for the report.

[Tim Graham <timograham@…>]

In d8961a5ad05b103b7b37e8e7a992f5d2f761fc2c:

[1.7.x] Fixed #22827 -- clarified what each cache middleware does.

Thanks Keryn Knight for the report.

Backport of f9b7a0383c from master