Fixed #24035 -- Clarified docs on CACHE_MIDDLEWARE_KEY_PREFIX vs KEY_PREFIX

Similar to above, `cache_page()` passes `key_prefix=None` to `CacheMiddleware` which results in an empty string being used instead of the setting: https://github.com/django/django/blob/227a4c48be18a439f0a08a49bf1f3062babeee82/django/middleware/cache.py#L167-L169