Docs/mm/damon/maintainer-profile: allow posting patches based on damon/next tree

The document mentions any patches for review should based on mm-unstable
instead of damon/next.  It should be the recommended process, but
sometimes patches based on damon/next could be posted for some reasons. 
Actually, the DAMON-based tiered memory management patchset[1] was written
on top of 'young page' DAMOS filter patchset, which was in damon/next tree
as of the writing.

Allow such case and just ask such things to be clearly specified.

[1] https://lore.kernel.org/20240405060858.2818-1-honggyu.kim@sk.com

Link: https://lkml.kernel.org/r/20240503180318.72798-11-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>