Release notes update for non-BC changes

4.3.0 client will not be able to connect to 4.2.0 servers, due to something stupid in the protocol version handling. 4.2.0 and lower will blindly reject any messages which have a protocolVersion higher than that it's current version, even if the data format has not changed. The release notes should reflect this. Also, think about how the protocolVersion is used some more.