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

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

Link: https://lkml.kernel.org/r/20221119005215.3052436-4-shr@devkernel.io


Signed-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>