cgroup: Add documentation for missing zswap memory.stat

This includes zswpin, zswpout and zswpwb.

Signed-off-by: Usama Arif <usamaarif642@gmail.com>
Acked-by: Nhat Pham <nphamcs@gmail.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20240502185307.3942173-2-usamaarif642@gmail.com>