Fixed #24035 -- Clarified docs on CACHE_MIDDLEWARE_KEY_PREFIX vs KEY_PREFIX

Actually this doesn't seem to be accurate as `cache_page()` passes `cache_alias=None` to `CacheMiddleware` which results in `DEFAULT_CACHE_ALIAS` being used instead of the setting: https://github.com/django/django/blob/227a4c48be18a439f0a08a49bf1f3062babeee82/django/middleware/cache.py#L175-L177