Add javadoc to Handler.Callback.handleMessage

Describing param and return value. There is a public bug for this at:
https://code.google.com/p/android/issues/detail?id=6464

Change-Id: Ia917cd4e047b629e6511bf12a91a5e953da8ae2e

core/java/android/os/Handler.java

@@ -73,6 +73,9 @@ public class Handler {
     /**
      * Callback interface you can use when instantiating a Handler to avoid
      * having to implement your own subclass of Handler.
+     *
+     * @param msg A {@link android.os.Message Message} object
+     * @return True if no further handling is desired
      */
     public interface Callback {
         public boolean handleMessage(Message msg);