mm/damon/core: document damos_commit_dests() failure semantics

mm/damon/core: document damos_commit_dests() failure semantics

Add a kernel-doc-like comment to damos_commit_dests() documenting its
allocation failure contract: on -ENOMEM, the destination structure is left
in a partially torn-down state that is safe to deallocate via
damon_destroy_scheme(), but must not be reused for further commits.

This was unclear from the code alone and led to a separate patch [1]
attempting to reset nr_dests on failure.  Make the intended usage explicit
so future readers do not repeat the confusion.

Link: https://lkml.kernel.org/r/20260320143648.91673-1-sj@kernel.org
Link: https://lore.kernel.org/20260318214939.36100-1-objecting@objecting.org
Signed-off-by: Josh Law <objecting@objecting.org>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>