Fixing Android notification options descriptions (#820)

* Fixing doc bug that conflated sound and tag options for Android notifications.

* Removing duplicate documentation for tag.

* Adding tag details in the right place this time, hopefully.

Author: egilmorez

src/index.d.ts

@@ -4659,7 +4659,6 @@ declare namespace admin.messaging {
    * for code samples and detailed documentation.
    */
   interface NotificationMessagePayload {
-    tag?: string;
 
     /**
      * The notification's body text.
@@ -4699,6 +4698,15 @@ declare namespace admin.messaging {
      */
     color?: string;
 
+    /**
+     * The sound to play when the device receives the notification. Supports
+     * "default" or the filename of a sound resource bundled in the app. Sound files
+     * must reside in `/res/raw/`.
+     * 
+     * **Platforms:** Android
+     */
+    sound?: string;
+
     /**
      * Identifier used to replace existing notifications in the notification drawer.
      *
@@ -4708,8 +4716,8 @@ declare namespace admin.messaging {
      * the new notification replaces the existing one in the notification drawer.
      *
      * **Platforms:** Android
-     */
-    sound?: string;
+     */ 
+    tag?: string;
 
     /**
      * The notification's title.