Docs/ABI/damon: document DAMOS filter allow sysfs file

Update DAMON ABI document for added DAMOS filter 'allow' file.

Link: https://lkml.kernel.org/r/20250109175126.57878-9-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>