402726 - WebAppContext references old WebSocket packages in system and server classes

+ Removing redundant WebSocketServlet references
 + Changing comments to be more clear about the roles of the 2 remaining
   entries

jetty-webapp/src/main/java/org/eclipse/jetty/webapp/WebAppContext.java

@@ -115,8 +115,7 @@ public class WebAppContext extends ServletContextHandler implements WebAppClassL
         "org.eclipse.jetty.continuation.",  // webapp cannot change continuation classes
         "org.eclipse.jetty.jndi.",          // webapp cannot change naming classes
         "org.eclipse.jetty.jaas.",          // webapp cannot change jaas classes
-        "org.eclipse.jetty.websocket.",        // WebSocket is a jetty extension
-        "org.eclipse.jetty.websocket.WebSocketServlet", // webapp cannot change WebSocketServlet
+        "org.eclipse.jetty.websocket.",     // webapp cannot change / replace websocket classes
         "org.eclipse.jetty.servlet.DefaultServlet" // webapp cannot change default servlets
     } ;
 
@@ -130,10 +129,9 @@ public class WebAppContext extends ServletContextHandler implements WebAppClassL
         "-org.eclipse.jetty.jndi.",         // don't hide naming classes
         "-org.eclipse.jetty.jaas.",         // don't hide jaas classes
         "-org.eclipse.jetty.servlets.",     // don't hide jetty servlets
-        "-org.eclipse.jetty.websocket.WebSocketServlet", // don't hide WebSocketServlet
         "-org.eclipse.jetty.servlet.DefaultServlet", // don't hide default servlet
         "-org.eclipse.jetty.servlet.listener.", // don't hide useful listeners
-        "-org.eclipse.jetty.websocket.",    //not a serverclass, is a systemclass
+        "-org.eclipse.jetty.websocket.",    // don't hide websocket classes from webapps (allow webapp to use ones from system classloader)
         "org.eclipse.jetty."                // hide other jetty classes
     } ;