mm/ksm: document pages_skipped sysfs knob

This adds documentation for the new metric pages_skipped.

Link: https://lkml.kernel.org/r/20230926040939.516161-5-shr@devkernel.ioSigned-off-by: Stefan Roesch <shr@devkernel.io>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Rik van Riel <riel@surriel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/admin-guide/mm/ksm.rst

@@ -178,6 +178,8 @@ pages_unshared
         how many pages unique but repeatedly checked for merging
 pages_volatile
         how many pages changing too fast to be placed in a tree
+pages_skipped
+        how many pages did the "smart" page scanning algorithm skip
 full_scans
         how many times all mergeable areas have been scanned
 stable_node_chains