Docs/ABI/damon: document nid file

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

Link: https://lkml.kernel.org/r/20250420194030.75838-7-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>