title: How to add String Extension on Apple DocC
body: <p>I am checking new Apple documentation on <code>Xcode 13 beta</code>. I am able to <code>build documentation</code> with DocC.</p>
  <p>But I am not able to see any documentation created for extensions like String extension, Date extension.</p>
  <p>As per example for below case it will not add any documentation.</p>
  <pre><code>extension String {
  
      /// Description for DocC
      func myMethod() {
         
      }
  }
  </code></pre>
  <p>Consider, If I have created an extension for any struct or class, then it is adding documentation like below code</p>
  <pre><code>public struct MyStruct {
  
  }
  
  
  extension MyStruct {
  
    /// Description
    func myMethod() {
   
    }
  }
  </code></pre>
  <p>Is this default behaviour by apple or I am missing anything?</p>