DOCSP-17736 cluster chaining parameter (#1305)

Co-authored-by: jason-price-mongodb <[email protected]>

Author: jeff-allen-mongo

source/reference/parameters.txt

@@ -1567,6 +1567,33 @@ The following parameters support diagnostic data capture (FTDC):
 Logical Session Parameters
 ~~~~~~~~~~~~~~~~~~~~~~~~~~
 
+.. parameter:: enableOverrideClusterChainingSetting
+
+   .. versionadded:: 4.2.16
+
+   |both|
+
+   *Type*: boolean
+
+   *Default*: false
+   
+   If :parameter:`enableOverrideClusterChainingSetting` is ``true``,
+   replica set :term:`secondary` members can replicate data from
+   other secondary members even if :rsconf:`settings.chainingAllowed` is
+   ``false``.
+
+   You can only set :parameter:`enableOverrideClusterChainingSetting` at
+   startup and cannot change this setting with the
+   :dbcommand:`setParameter` command.
+
+   For example, to set the
+   :parameter:`enableOverrideClusterChainingSetting` for a
+   :binary:`~bin.mongod` instance to ``true``:
+
+   .. code-block:: bash
+
+      mongod --setParameter enableOverrideClusterChainingSetting=true
+
 .. parameter:: logicalSessionRefreshMillis
 
    .. admonition:: Availability

source/reference/replica-configuration.txt

@@ -400,11 +400,27 @@ Replica Set Configuration Fields
 
       *Default*: true
 
-      When :rsconf:`settings.chainingAllowed` is
-      ``true``, the replica set allows :term:`secondary` members to
-      replicate from other secondary members. When
-      :rsconf:`settings.chainingAllowed` is
-      ``false``, secondaries can replicate only from the :term:`primary`.
+      In MongoDB 4.2.15 and earlier, if
+      :rsconf:`settings.chainingAllowed` is:
+      
+      - ``true``, replica set :term:`secondary` members can
+        replicate data from other secondary members.
+      
+      - ``false``, secondary members can replicate data only from the
+        :term:`primary`.
+
+      Starting in MongoDB 4.2.16:
+      
+      - Replica set :term:`secondary` members can
+        replicate data from other secondary members even if
+        :rsconf:`settings.chainingAllowed` is ``false``.
+      
+      - To override :rsconf:`settings.chainingAllowed`, set the
+        :parameter:`enableOverrideClusterChainingSetting` server
+        parameter to ``true``.
+
+      - The default for
+        :parameter:`enableOverrideClusterChainingSetting` is ``false``.
 
       .. seealso:: :doc:`/tutorial/manage-chained-replication`