Add a comment explaning the logic

Author: lahirumaramba

firebase_admin/_messaging_encoder.py

@@ -324,6 +324,8 @@ def encode_android_notification(cls, notification):
 
         event_time = result.get('event_time')
         if event_time:
+            # if the datetime instance is not naive (tzinfo is present), convert to UTC
+            # otherwise (tzinfo is None) assume the datetime instance is already in UTC
             if event_time.tzinfo is not None:
                 event_time = event_time.astimezone(datetime.timezone.utc)
             result['event_time'] = event_time.strftime('%Y-%m-%dT%H:%M:%S.%fZ')