Docs/admin-guide/mm/damon/usage: clarify quotas and watermarks sysfs interface

Explanation of DAMOS quotas and watermarks are not clearly explaining the
meaning and expectation of each file.  Add more clarification for those.

Link: https://lkml.kernel.org/r/20230616191742.87531-7-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>