In the documentation we say that:

pv1 is the default for all new replica sets created with MongoDB 3.2 or later

It can be read as if protocolVersion is not set explicitly it would mean that pv1 is used.

We should clarify that pv1 is only in effect when protocolVersion is set to 1 in the replica set configuration. From that, the default means that when rs.initiate() is executed on 3.2 or newer without protocolVersion being set, the server would automatically set protocolVersion in the replica set configuration to 1.

We should also explain how to check the current protocol version - that is the user must inspect the replica set configuration to see if protocolVersion is set or not. If it's not set or set to 0 that would mean pv0, if it's set to 1 - that's pv1.