Docs/admin-guide/mm/zswap: remove a paragraph about zswap being a new feature

Nine years have passed since Linux 3.11.

Signed-off-by: David Heidelberg <david@ixit.cz>
Link: https://lore.kernel.org/r/20221104122612.14906-1-david@ixit.cz


Signed-off-by: Jonathan Corbet <corbet@lwn.net>