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.comSigned-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>

Documentation/admin-guide/cgroup-v2.rst

@@ -1755,6 +1755,8 @@ The following nested keys are defined.
 
 	Note that this is subtly different from setting memory.swap.max to
 	0, as it still allows for pages to be written to the zswap pool.
+	This setting has no effect if zswap is disabled, and swapping
+	is allowed unless memory.swap.max is set to 0.
 
   memory.pressure
 	A read-only nested-keyed file.