Update documentation - use with webSocket only

[rubroboletus]

Dependencies

none

Feature Request

In some environments, it is impossible to use jenkins agents with direct JNLP connection. But it is possible to use "webSocket". You just need to modify the docker agent template command like this one:

sh -cx curl --connect-timeout 20 --max-time 60 -o agent.jar $DOCKER_SWARM_PLUGIN_JENKINS_AGENT_JAR_URL && java -cp agent.jar hudson.remoting.jnlp.Main -headless -url `echo $DOCKER_SWARM_PLUGIN_JENKINS_AGENT_JAR_URL | sed 's/jnlpJars\/agent.jar//'` -workDir /tmp -webSocket $DOCKER_SWARM_PLUGIN_JENKINS_AGENT_SECRET $DOCKER_SWARM_PLUGIN_JENKINS_AGENT_NAME

Please update documentation.