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

Documentation/admin-guide/cgroup-v2.rst

@@ -1684,9 +1684,9 @@ per-cgroup dirty memory states are examined and the more restrictive
 of the two is enforced.
 
 cgroup writeback requires explicit support from the underlying
-filesystem.  Currently, cgroup writeback is implemented on ext2, ext4
-and btrfs.  On other filesystems, all writeback IOs are attributed to
-the root cgroup.
+filesystem.  Currently, cgroup writeback is implemented on ext2, ext4,
+btrfs, f2fs, and xfs.  On other filesystems, all writeback IOs are 
+attributed to the root cgroup.
 
 There are inherent differences in memory and writeback management
 which affects how cgroup ownership is tracked.  Memory is tracked per