mm: document /sys/class/bdi/<bdi>/max_bytes knob

This documents the new /sys/class/bdi/<bdi>/max_bytes knob.

Link: https://lkml.kernel.org/r/20221119005215.3052436-10-shr@devkernel.ioSigned-off-by: Stefan Roesch <shr@devkernel.io>
Cc: Chris Mason <clm@meta.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/ABI/testing/sysfs-class-bdi

@@ -56,6 +56,20 @@ Description:
 	be trusted to play fair.
 
 	(read-write)
+
+What:		/sys/class/bdi/<bdi>/max_bytes
+Date:		October 2022
+Contact:	Stefan Roesch <shr@devkernel.io>
+Description:
+	Allows limiting a particular device to use not more than the
+	given 'max_bytes' of the write-back cache.  This is useful in
+	situations where we want to avoid one device taking all or
+	most of the write-back cache.  For example in case of an NFS
+	mount that is prone to get stuck, a FUSE mount which cannot be
+	trusted to play fair, or a nbd device.
+
+	(read-write)
+
 What:		/sys/class/bdi/<bdi>/strict_limit
 Date:		October 2022
 Contact:	Stefan Roesch <shr@devkernel.io>