Docs/admin-guide/mm/damon/usage: document empty target regions commit behavior

Docs/admin-guide/mm/damon/usage: document empty target regions commit behavior

Committing a monitoring target with empty target regions is for keeping
the current monitoring results.  This behavior was introduced by commit
973233600676 ("mm/damon/sysfs: update monitoring target regions for online
input commit").  The behavior is not documented, though.  Update the usage
document for clarifying this behavior.

Link: https://lkml.kernel.org/r/20251026182216.118200-5-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>