Here is the link of the document in question
1. http://docs.mongodb.org/manual/tutorial/configure-secondary-only-replica-set-member/#prevent-secondary-from-becoming-primary

The document uses a four member replica set example. This can be misleading to users who are new to MongoDB. They may think it is OK to have a four nodes replica set; all they have to do is to adjust the node priority.

Please make it a five member example instead of four or specify that there is an arbiter in addition to the 4 members.