stub: add docs clarifying readiness

Author: carl-mastrangelo

core/src/main/java/io/grpc/ClientCall.java

@@ -141,6 +141,10 @@ public void onClose(Status status, Metadata trailers) {}
      * {@link #sendMessage}) without requiring excessive buffering internally. This event is
      * just a suggestion and the application is free to ignore it, however doing so may
      * result in excessive buffering within the ClientCall.
+     *
+     * <p>If the type of a call is either {@link MethodDescriptor.MethodType#UNARY} or
+     * {@link MethodDescriptor.MethodType#SERVER_STREAMING}, this callback may not be fired. Calls
+     * that send exactly one message should not await this callback.
      */
     public void onReady() {}
   }
@@ -223,6 +227,10 @@ public void onReady() {}
    *
    * <p>This abstract class's implementation always returns {@code true}. Implementations generally
    * override the method.
+   *
+   * <p>If the type of the call is either {@link MethodDescriptor.MethodType#UNARY} or
+   * {@link MethodDescriptor.MethodType#SERVER_STREAMING}, this method may persistently return
+   * false. Calls that send exactly one message should not check this method.
    */
   public boolean isReady() {
     return true;