DOCS-14330 Add deployment blurb back to shardedcluster page (mongodb#2363)

* DOCS-14330 Add deployment blurb back to shardedcluster page

* internal feedback

* XR feedback 1

Author: ianf-mongodb

source/core/sharded-cluster-components.txt

@@ -66,10 +66,13 @@ Number of ``mongos`` and Distribution
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Deploying multiple :binary:`~bin.mongos` routers supports high
-availability and scalability. For shard-level high availability, a
-common pattern is to place :binary:`~bin.mongos` instances on the same
-hardware that :binary:`~bin.mongod` instances are already running on.
-Another option is to embed :binary:`~bin.mongos` routers with
+availability and scalability. If a proxy or load balancer is between the 
+application and the ``mongos`` routers, you must configure it for 
+:term:`client affinity`. Client affinity allows every connection from a 
+single client to reach the same ``mongos``. For shard-level high 
+availability, a common pattern is to place ``mongos`` instances on the 
+same hardware that ``mongos`` instances are already running on.
+Another option is to embed ``mongos`` routers with
 application tier infrastructure.
 
 There is no limit to the number of :binary:`~bin.mongos` routers you can

source/reference/glossary.txt

@@ -168,6 +168,9 @@ Glossary
 
       Client can also refer to a single thread or process.
 
+   client affinity
+      A consistent client connection to a specified data source.
+
    cluster
       See :term:`sharded cluster`.