mm/damon: document non-zero length damon_region assumption

mm/damon: document non-zero length damon_region assumption

DAMON regions are assumed to always be non-zero length.  There was a
confusion [1] about it, probably due to lack of the documentation.
Document it.

Link: https://lkml.kernel.org/r/20260307195356.203753-5-sj@kernel.org
Link: https://lore.kernel.org/20251231070029.79682-1-sj@kernel.org/
Signed-off-by: SeongJae Park <sj@kernel.org>
Acked-by: wang lian <lianux.mm@gmail.com>
Cc: Brendan Higgins <brendan.higgins@linux.dev>
Cc: David Gow <davidgow@google.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>