[1.6.x] Fixed #23762 -- clarified CACHE_MIDDLEWARE_ANONYMOUS_ONLY deprecation in docs

``CACHE_MIDDLEWARE_ANONYMOUS_ONLY`` setting

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

``CacheMiddleware`` used to provide a way to cache requests only if they

weren't made by a logged-in user. This mechanism was largely ineffective

because the middleware correctly takes into account the ``Vary: Cookie`` HTTP

header, and this header is being set on a variety of occasions, such as:

``CacheMiddleware`` and ``UpdateCacheMiddleware`` used to provide a way to

cache requests only if they weren't made by a logged-in user. This mechanism

was largely ineffective because the middleware correctly takes into account the

``Vary: Cookie`` HTTP header, and this header is being set on a variety of

occasions, such as:

* accessing the session, or

* using CSRF protection, which is turned on by default, or