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

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

Commit cc7ceb1d14b0 ("mm/damon/stat: expose the current tuned aggregation
interval"), has introduced 'aggr_interval_us' parameter for DAMON_STAT.
But the new parameter is not yet documented.  Document it on the usage
document for the module.

Link: https://lkml.kernel.org/r/20251026182216.118200-8-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>