Add more descriptive comment to the GCM fallback parameter

The comment for the GCM fallback should be more descriptive and
therefore the comment was updated to be more descriptive.

app/config/parameters.yml.dist

@@ -38,7 +38,8 @@ parameters:
     # PCRE as accepted by preg_match (http://php.net/preg_match).
     mobile_app_user_agent_pattern: "/^.*$/"
 
-    # Flag for GCM fallback with FCM on failure
+    # Flag to use Firebse as fallback when GCM fails with a MismatchSenderId.
+    # This is used to handle the push notification method rollover from GCM to Firebase.
     use_firebase_fallback_for_gcm: true
 
     # Options for the tiqr library