[Cronet] Better document Experimental Cronet classes

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

Index: components/cronet/android/api/src/org/chromium/net/ExperimentalCronetEngine.java
diff --git a/components/cronet/android/api/src/org/chromium/net/ExperimentalCronetEngine.java b/components/cronet/android/api/src/org/chromium/net/ExperimentalCronetEngine.java
index 6d034ad1314621c3194827b479e625f156ddb3b4..d1a14b26903a7489fc01e573b5fa9bafcba9ea74 100644
--- a/components/cronet/android/api/src/org/chromium/net/ExperimentalCronetEngine.java
+++ b/components/cronet/android/api/src/org/chromium/net/ExperimentalCronetEngine.java
@@ -15,9 +15,14 @@ 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.
+ * Instances of this class are not meant for general use, but instead only
+ * to access experimental features. Note that experimental features may be
+ * transitioned to supported features (and moved from
+ * {@link ExperimentalCronetEngine} to {@code CronetEngine}) or may be
+ * unsupported, in which case calling them may have no effect.
  *
  * {@hide since this class exposes experimental features that should be hidden.}
  */
@@ -70,7 +75,11 @@ public abstract class ExperimentalCronetEngine extends CronetEngine {
     public static final int EFFECTIVE_CONNECTION_TYPE_4G = 5;
 
     /**
-     * Builder for building {@link ExperimentalCronetEngine}.
+     * A version of {@link CronetEngine.Builder} that exposes experimental
+     * features. Note that experimental features may be
+     * transitioned to supported features (and moved from
+     * {@link ExperimentalCronetEngine.Builder} to {@code CronetEngine.Builder})
+     * or may be unsupported, in which case calling them may have no effect.
      */
     public static class Builder extends CronetEngine.Builder {
         /**