Versioned JavaDocs for Apache ActiveMQ could be found at
http://activemq.apache.org/maven/x.y.z/apidocs/index.html kind of url, where x.y.z is broker version you're looking for.
For example http://activemq.apache.org/maven/5.12.1/apidocs/index.html contains JavaDocs for 5.12.1.