Docs/mm/damon/design: update for max_nr_snapshots

Update DAMON design document for the newly added snapshot level DAMOS
deactivation feature, max_nr_snapshots.

Link: https://lkml.kernel.org/r/20251216080128.42991-10-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/mm/damon/design.rst

@@ -720,6 +720,7 @@ scheme's execution.
 - ``qt_exceeds``: Total number of times the quota of the scheme has exceeded.
 - ``nr_snapshots``: Total number of DAMON snapshots that the scheme is tried to
   be applied.
+- ``max_nr_snapshots``: Upper limit of ``nr_snapshots``.
 
 "A scheme is tried to be applied to a region" means DAMOS core logic determined
 the region is eligible to apply the scheme's :ref:`action
@@ -741,6 +742,10 @@ to exclude anonymous pages and the region has only anonymous pages, or if the
 action is ``pageout`` while all pages of the region are unreclaimable, applying
 the action to the region will fail.
 
+Unlike normal stats, ``max_nr_snapshots`` is set by users.  If it is set as
+non-zero and ``nr_snapshots`` be same to or greater than ``nr_snapshots``, the
+scheme is deactivated.
+
 To know how user-space can read the stats via :ref:`DAMON sysfs interface
 <sysfs_interface>`, refer to :ref:s`stats <sysfs_stats>` part of the
 documentation.