Docs/admin-guide/mm/zswap: remove zsmalloc's lack of writeback warning

Docs/admin-guide/mm/zswap: remove zsmalloc's lack of writeback warning

Writeback has been implemented for zsmalloc, so this warning no longer
holds.

Link: https://lkml.kernel.org/r/20230106220016.172303-1-nphamcs@gmail.com
Fixes: 9997bc017549a ("zsmalloc: implement writeback mechanism for zsmalloc")
Suggested-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Nhat Pham <nphamcs@gmail.com>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>