Documentation/cgroup-v2: clarify that zswap.writeback is ignored if zswap is disabled

As discussed in [1], zswap-related settings natually lose their effect
when zswap is disabled, specifically zswap.writeback here.  Be explicit
about this behavior.

[1] https://lore.kernel.org/linux-kernel/CAKEwX=Mhbwhh-=xxCU-RjMXS_n=RpV3Gtznb2m_3JgL+jzz++g@mail.gmail.com/

[akpm@linux-foundation.org: fix/simplify text]
Link: https://lkml.kernel.org/r/20240823162506.12117-3-me@yhndnzj.com


Signed-off-by: Mike Yuan <me@yhndnzj.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michal Koutný <mkoutny@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>