Warn about secondaryPreferred possibly redirecting too much traffic to primary

Author: A. Jesse Jiryu Davis

source/applications/replication.txt

@@ -621,6 +621,16 @@ using non-:readmode:`primary` read preference modes:
   direct connection to a :ref:`hidden member
   <replica-set-hidden-members>` of the set.
 
+.. warning::
+
+  Although it is tempting to use :readmode:`secondaryPreferred` to distribute
+  load, if your primary cannot accept your full read load, then
+  using :readmode:`secondaryPreferred` is dangerous: if all secondaries
+  become unavailable, but you have enough arbiters to prevent the primary from
+  stepping down, then this mode will redirect traffic to the primary. For this
+  reason, use :readmode:`secondary` to distribute load,
+  not :readmode:`secondaryPreferred`.
+
 - Providing local reads for geographically distributed applications.
 
   If you have application servers in multiple data centers, you may