Docs/admin-guide/mm/damon/usage: document 'nid' file

Add description of 'nid' file, which is optionally used for specific DAMOS
quota goal metrics such as node_mem_{used,free}_bp on DAMON usage
document.

Link: https://lkml.kernel.org/r/20250420194030.75838-6-sj@kernel.org


Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Yunjeong Mun <yunjeong.mun@sk.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>