DOCS-15182 BACKPORT add loadRoutingTableOnStartup (#997)

* DOCS-15182 BACKPORT loadRoutingTableOnStartup

* Staging fixes

Author: Dave

source/reference/parameters.txt

@@ -2578,6 +2578,38 @@ Sharding Parameters
       - :parameter:`ShardingTaskExecutorPoolMinSize`
 
 
+.. parameter:: loadRoutingTableOnStartup
+
+   .. versionadded:: 4.2.20
+
+   Type: boolean
+
+   *Default*: true
+
+   |mongos-only|
+
+   Configures a :binary:`~bin.mongos` instance to preload the routing
+   table for a sharded cluster on startup. With this setting
+   enabled, the :binary:`~bin.mongos` caches the cluster-wide routing
+   table for each sharded collection as part of its startup procedure,
+   before it begins accepting client connections.
+
+   Without this setting enabled, the :binary:`~bin.mongos` only loads
+   a routing table as needed for incoming client connections, and only
+   loads the specific routing table for the namespace of a given
+   request.
+
+   A :binary:`~bin.mongos` instance with the
+   :parameter:`loadRoutingTableOnStartup` parameter enabled may
+   experience longer startup times, but will result in faster servicing
+   of initial client connections once started.
+
+   :parameter:`loadRoutingTableOnStartup` is enabled by default.
+
+   You can only set this parameter on startup, use the
+   :setting:`setParameter` configuration file setting or the
+   :option:`--setParameter <mongos --setParameter>` command line option.
+
 .. parameter:: migrateCloneInsertionBatchDelayMS
 
    .. versionadded:: 4.0.5