trivial: add headings to setReadPref

Author: kay-kim

source/reference/method/Mongo.setReadPref.txt

@@ -51,12 +51,20 @@ Definition
 Examples
 --------
 
-To set a read preference mode in the :binary:`~bin.mongo` shell, use the
-following operation:
+
+Specify Read Preference Mode
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The following operation in the :binary:`~bin.mongo` shell sets the read
+preference :ref:`mode <mongo-setReadPref-mode>` to allow reads from
+secondary members and target the read to a secondary member.
 
 .. code-block:: javascript
 
-   db.getMongo().setReadPref('primaryPreferred')
+   db.getMongo().setReadPref('secondary')
+
+Specify Read Preference Tag Set
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 To set a read preference that uses a tag set, specify an array of tag
 sets as the second argument to :method:`Mongo.setReadPref()`, as in the