docs/vm: ksm: update stable_node_chains_prune_millisecs description

Make the description of stable_node_chains_prune_millisecs sysfs parameter
less implementation aware and add a few words about this parameter in the
"Design" section.

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/vm/ksm.rst

@@ -150,14 +150,12 @@ max_page_sharing
         traversals are always schedule friendly themselves.
 
 stable_node_chains_prune_millisecs
-        How frequently to walk the whole list of stable_node "dups"
-        linked in the stable_node "chains" in order to prune stale
-        stable_nodes. Smaller milllisecs values will free up the KSM
-        metadata with lower latency, but they will make ksmd use more
-        CPU during the scan. This only applies to the stable_node
-        chains so it's a noop if not a single KSM page hit the
-        ``max_page_sharing`` yet (there would be no stable_node chains in
-        such case).
+        specifies how frequently KSM checks the metadata of the pages
+        that hit the deduplication limit for stale information.
+        Smaller milllisecs values will free up the KSM metadata with
+        lower latency, but they will make ksmd use more CPU during the
+        scan. It's a noop if not a single KSM page hit the
+        ``max_page_sharing`` yet.
 
 The effectiveness of KSM and MADV_MERGEABLE is shown in ``/sys/kernel/mm/ksm/``:
 
@@ -249,6 +247,11 @@ deduplication factor at the expense of slower worst case for rmap
 walks for any KSM page which can happen during swapping, compaction,
 NUMA balancing and page migration.
 
+The whole list of stable_node "dups" linked in the stable_node
+"chains" is scanned periodically in order to prune stale stable_nodes.
+The frequency of such scans is defined by
+``stable_node_chains_prune_millisecs`` sysfs tunable.
+
 Reference
 ---------
 .. kernel-doc:: mm/ksm.c