(DOCSP-15892): Read preference difference between mongosh and mongo (#107)

* (DOCSP-15892): Read preference difference between mongosh and mongo

* Tweaks per feedback

* typo

* reorder

Author: jeff-allen-mongo

source/reference/compatibility.txt

@@ -52,3 +52,44 @@ the methods listed in the :guilabel:`Alternative Methods` column.
        - :method:`db.collection.findOneAndUpdate()`
        - :method:`db.collection.bulkWrite()`
 
+Read Preference Behavior
+------------------------
+
+Read Operations on a Secondary Node
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+When using ``mongosh`` to connect directly to a :term:`secondary`
+replica set member, you do not have to explicitly enable secondary reads
+with ``rs.secondaryOk()``.
+
+You can read from a secondary member if you specify a
+:manual:`read preference </core/read-preference/>` of either:
+
+- :readmode:`primaryPreferred`
+- :readmode:`secondary`
+- :readmode:`secondaryPreferred`
+
+To specify a read preference, you can use either:
+
+- The :urioption:`readPreference` connection string option when
+  connecting to the node.
+
+- The :method:`Mongo.setReadPref()` method.
+
+The legacy ``mongo`` shell required running ``rs.secondaryOk()`` to
+enable secondary reads.
+
+``show`` Helper Methods
+~~~~~~~~~~~~~~~~~~~~~~~
+
+The following ``show`` helper methods always use a read preference of
+``primaryPreferred``, even when a different read preference has been
+specified for the operation:
+
+- ``show dbs``
+- ``show databases``
+- ``show collections``
+- ``show tables``
+
+In the legacy ``mongo`` shell, these operations use the specified read
+preference.