Update doc string for dryRun in FCM send methods. (#509)

* Update doc string for dryRun in FCM send methods to mention that it cannot be used to validate APNS tokens.

Author: chong-shao

src/main/java/com/google/firebase/messaging/FirebaseMessaging.java

@@ -90,7 +90,9 @@ public String send(@NonNull Message message) throws FirebaseMessagingException {
    * Sends the given {@link Message} via Firebase Cloud Messaging.
    *
    * <p>If the {@code dryRun} option is set to true, the message will not be actually sent. Instead
-   * FCM performs all the necessary validations, and emulates the send operation.
+   * FCM performs all the necessary validations and emulates the send operation. The {@code dryRun}
+   * option is useful for determining whether an FCM registration has been deleted. However, it 
+   * cannot be used to validate APNs tokens.
    *
    * @param message A non-null {@link Message} to be sent.
    * @param dryRun a boolean indicating whether to perform a dry run (validation only) of the send.
@@ -161,8 +163,10 @@ public BatchResponse sendAll(
    * send the entire list as a single RPC call. Compared to the {@link #send(Message)} method, this
    * is a significantly more efficient way to send multiple messages.
    *
-   * <p>If the {@code dryRun} option is set to true, the messages will not be actually sent. Instead
-   * FCM performs all the necessary validations, and emulates the send operation.
+   * <p>If the {@code dryRun} option is set to true, the message will not be actually sent. Instead
+   * FCM performs all the necessary validations, and emulates the send operation. The {@code dryRun}
+   * option is useful for determining whether an FCM registration has been deleted. But it cannot be
+   * used to validate APNs tokens.
    *
    * <p>The responses list obtained by calling {@link BatchResponse#getResponses()} on the return
    * value corresponds to the order of input messages.
@@ -227,7 +231,9 @@ public BatchResponse sendMulticast(
    * Sends the given multicast message to all the FCM registration tokens specified in it.
    *
    * <p>If the {@code dryRun} option is set to true, the message will not be actually sent. Instead
-   * FCM performs all the necessary validations, and emulates the send operation.
+   * FCM performs all the necessary validations, and emulates the send operation. The {@code dryRun}
+   * option is useful for determining whether an FCM registration has been deleted. But it cannot be
+   * used to validate APNs tokens.
    *
    * <p>This method uses the {@link #sendAll(List)} API under the hood to send the given
    * message to all the target recipients. The responses list obtained by calling