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.io
Signed-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>