Docs/admin-guide/mm/damon/stat: document kdamond_pid parameter

Docs/admin-guide/mm/damon/stat: document kdamond_pid parameter

Update DAMON_STAT usage document for newly added kdamond_pid parameter.

Link: https://lore.kernel.org/20260502020505.80822-3-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>