[Cronet] Better document Experimental Cronet classes

components/cronet/android/api/src/org/chromium/net/ExperimentalCronetEngine.java

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 package org.chromium.net;
 
 import android.content.Context;
 import android.support.annotation.VisibleForTesting;
 
 import java.io.IOException;
 import java.net.Proxy;
 import java.net.URL;
 import java.net.URLConnection;
 import java.util.Date;
 import java.util.Set;
 import java.util.concurrent.Executor;
 
 /**
- * {@link CronetEngine} that exposes experimental features. Use {@link Builder} to build an
- * instance of this class. Every instance of {@link CronetEngine} can be casted to an instance
- * of this class.
+ * {@link CronetEngine} that exposes experimental features. To obtain an
+ * instance of this class, cast a {@code CronetEngine} to this type. Every
+ * instance of {@code CronetEngine} can be cast to an instance of this class,
+ * as they are backed by the same implementation and hence perform identically.
+ * Instances of this class are not meant for general use, but instead only
+ * to access experimental features. Experimental features may be deprecated in the
+ * future. Use at your own risk.
  *
  * {@hide since this class exposes experimental features that should be hidden.}
  */
 public abstract class ExperimentalCronetEngine extends CronetEngine {
     /**
      * The value of a connection metric is unknown.
      */
     public static final int CONNECTION_METRIC_UNKNOWN = -1;
 
     /**
@@ skipping 32 matching lines @@
     public static final int EFFECTIVE_CONNECTION_TYPE_3G = 4;
 
     /**
      * The estimate of the effective connection type is 4G.
      *
      * @see #getEffectiveConnectionType
      */
     public static final int EFFECTIVE_CONNECTION_TYPE_4G = 5;
 
     /**
-     * Builder for building {@link ExperimentalCronetEngine}.
+     * A version of {@link CronetEngine.Builder} that exposes experimental
+     * features. Instances of this class are not meant for general use, but
+     * instead only to access experimental features. Experimental features
+     * may be deprecated in the future. Use at your own risk.
      */
     public static class Builder extends CronetEngine.Builder {
         /**
          * Constructs a {@link Builder} object that facilitates creating a
          * {@link CronetEngine}. The default configuration enables HTTP/2 and
          * disables QUIC, SDCH and the HTTP cache.
          *
          * @param context Android {@link Context}, which is used by
          *                {@link Builder} to retrieve the application
          *                context. A reference to only the application
@@ skipping 351 matching lines @@
      * {@link #CONNECTION_METRIC_UNKNOWN} if the value is
      * unavailable. This must be called after
      * {@link Builder#enableNetworkQualityEstimator}, and will
      * throw an exception otherwise.
      * @return Estimate of the downstream throughput in kilobits per second.
      */
     public int getDownstreamThroughputKbps() {
         return CONNECTION_METRIC_UNKNOWN;
     }
 }