A use-case for read preference mode primaryPreferred

Author: ajdavis

source/applications/replication.txt

@@ -639,6 +639,13 @@ using non-:readmode:`primary` read preference modes:
   read preference or the :readmode:`nearest` to avoid network
   latency.
 
+- Maintaining availability during a failover.
+
+  Use :readmode:`primaryPreferred` if you want your application to do
+  consistent reads from the primary under normal circumstances, but to allow
+  stale reads from secondaries in an emergency. This provides a "read-only mode"
+  for your application during a failover.
+
 Using read modes other than :readmode:`primary` and
 :readmode:`primaryPreferred` to provide extra capacity is not in and
 of itself justification for non-:readmode:`primary` in many