Fix send_all & send_multicast snippet comment to match implementation (#376)

* Fix send_all comment

* Update send_multicast comment

Author: alihen

snippets/messaging/cloud_messaging.py

@@ -225,7 +225,7 @@ def unsubscribe_from_topic():
 def send_all():
     registration_token = 'YOUR_REGISTRATION_TOKEN'
     # [START send_all]
-    # Create a list containing up to 100 messages.
+    # Create a list containing up to 500 messages.
     messages = [
         messaging.Message(
             notification=messaging.Notification('Price drop', '5% off all electronics'),
@@ -247,7 +247,7 @@ def send_all():
 
 def send_multicast():
     # [START send_multicast]
-    # Create a list containing up to 100 registration tokens.
+    # Create a list containing up to 500 registration tokens.
     # These registration tokens come from the client FCM SDKs.
     registration_tokens = [
         'YOUR_REGISTRATION_TOKEN_1',