Discourage usage of cache_page decorator with UpdateCacheMiddleware (or make middleware ignore decorated views)

[Serhiy]

If django.middleware.cache.UpdateCacheMiddleware is active and some view is decorated with the cache_page that overrides cache alias, this view will be cached in both caches, first time by cache_page and then by the middleware. Well, at least it will try. And fail if the default cache is memcached and the view response is bigger than its' maximum entry size. While filebased cache will work and even create the cache entry. This way first user to visit when there are no entry in cache will get error instead of data.
So, I guess there should be some warning about this in the docs.
Ideally, middleware should ignore views with overridden cache alias, maybe by means of checking headers like in case of the cache_control decorator.

[Carl Meyer]

Any attempt to fix this should also take into account #15855 -- the cache_page decorator is already basically broken in terms of Vary headers. It shouldn't cache at all, it should mark the response for later caching by the middleware, or something.