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

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

[akpm@linux-foundation.org: fix htmldocs warnings]
Link: https://lkml.kernel.org/r/20221119005215.3052436-15-shr@devkernel.ioSigned-off-by: NStefan Roesch <shr@devkernel.io>
Cc: Chris Mason <clm@meta.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>

Documentation/ABI/testing/sysfs-class-bdi

@@ -57,6 +57,21 @@ Description:
 
 	(read-write)
 
+What:		/sys/class/bdi/<bdi>/min_bytes
+Date:		October 2022
+Contact:	Stefan Roesch <shr@devkernel.io>
+Description:
+	Under normal circumstances each device is given a part of the
+	total write-back cache that relates to its current average
+	writeout speed in relation to the other devices.
+
+	The 'min_bytes' parameter allows assigning a minimum
+	percentage of the write-back cache to a particular device
+	expressed in bytes.
+	For example, this is useful for providing a minimum QoS.
+
+	(read-write)
+
 What:		/sys/class/bdi/<bdi>/max_bytes
 Date:		October 2022
 Contact:	Stefan Roesch <shr@devkernel.io>