title: $class syntax in jenkins scripted dsl
body: <p>Often when looking at scripted jenkins pipeline code, I see this pattern...</p>
  
  <pre><code>step([$class: 'GitHubSetCommitStatusBuilder',
          statusMessage: [content: 'Pipeline Started']])
  </code></pre>
  
  <p>I have not had any luck finding documentation on this technique and would love it if someone could explain what this is doing and when/why it is useful.  I believe this is a way to instantiate and populate the members of an underlying groovy class - but more detail would be appreciated.</p>
  
  <p>Also is this documented anywhere?</p>