Improved javadocs for onPreface().

jetty-http2/http2-common/src/main/java/org/eclipse/jetty/http2/api/Session.java

@@ -134,11 +134,18 @@ public interface Session
     public interface Listener
     {
         /**
-         * <p>Callback method invoked when the preface has been received.</p>
+         * <p>Callback method invoked:</p>
+         * <ul>
+         *     <li>for clients, just before the preface is sent, to gather the
+         *     SETTINGS configuration options the client wants to send to the server;</li>
+         *     <li>for servers, just after having received the preface, to gather
+         *     the SETTINGS configuration options the server wants to send to the
+         *     client.</li>
+         * </ul>
          *
          * @param session the session
          * @return a (possibly empty or null) map containing SETTINGS configuration
-         * options that are sent after the preface.
+         * options to send.
          */
         public Map<Integer, Integer> onPreface(Session session);