DOCSP-33087 Arbiters in Sharded Clusters (#5388) (#5477)

* DOCSP-33087 Arbiters in Sharded Clusters

* Fixes per Joe

* fixes per Joe

Author: kennethdyer

source/core/replica-set-arbiter.txt

@@ -38,6 +38,8 @@ Release Version Considerations
 
 .. include:: /includes/5.1/fact-do-not-use-with-quarterlies.rst
 
+.. _replica-set-arbiter-performance-psa:
+
 Performance Issues with PSA replica sets
 ----------------------------------------
 

source/core/sharded-cluster-shards.txt

@@ -16,8 +16,14 @@ A :term:`shard` contains a subset of sharded data for a :term:`sharded
 cluster`. Together, the cluster's shards hold the entire data set for the
 cluster.
 
-As of MongoDB 3.6, shards must be deployed as a :term:`replica set` to
-provide redundancy and high availability.
+Shards must be deployed as a :term:`replica set` to provide redundancy and high
+availability.
+
+.. important::
+
+   Sharded clusters use the write concern ``"majority"`` for a lot of internal
+   operations. Using an arbiter in a sharded cluster is discouraged due to
+   :ref:`replica-set-arbiter-performance-psa`.
 
 .. warning::