DOCS-12122: blurb about psa and cache pressure w read concern majority

Author: kay-kim

source/tutorial/add-replica-set-arbiter.txt

@@ -25,6 +25,24 @@ server or a monitoring host.
 Considerations
 --------------
 
+Read Concern ``majority`` and Three-Member PSA
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. admonition:: For 3-Member Primary-Secondary-Arbiter Architecture*
+   :class: warning
+
+   If you have a three-member replica set with a
+   primary-secondary-arbiter (PSA) architecture or a sharded cluster
+   with a three-member PSA shards, the cache pressure will increase if
+   any data bearing node is down and support for
+   :readconcern:`"majority"` read concern is enabled. 
+
+   To prevent the storage cache pressure from immobilizing a deployment
+   with a three-member primary-secondary-arbiter (PSA) architecture,
+   you can disable read concern "majority" for MongoDB 4.0.3+
+   and 3.6.1+. For more information, see
+   :ref:`4.0-disable-read-concern-majority`.
+
 Replica Set Protocol Version
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -40,6 +58,7 @@ process is added to the replica set, the arbiter will act like any other
 :binary:`~bin.mongod` process and start up with a set of data files and with a
 full-sized :term:`journal`.
 
+
 IP Binding
 ~~~~~~~~~~