Docs/admin-guide/mm/damon/usage: document DAMOS filter 'allow' sysfs file

Update DAMON usage document for the newly added 'allow' sysfs file for
DAMOS filters.

Link: https://lkml.kernel.org/r/20250109175126.57878-11-sj@kernel.org


Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>