doc: cgroup: add f2fs and xfs to supported list for writeback

f2fs and xfs have both added support for cgroup writeback:

578c6478 f2fs: implement cgroup writeback support
adfb5fb4 xfs: implement cgroup aware writeback

so add them to the supported list in the docs.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Link: https://lore.kernel.org/r/c8271324-9132-388c-5242-d7699f011892@redhat.com


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