docs: clarify that s3 can't access Glacier Vault

docs/content/s3.md

@@ -405,6 +405,9 @@ tries to access data from the glacier storage class you will see an error like b
 In this case you need to [restore](http://docs.aws.amazon.com/AmazonS3/latest/user-guide/restore-archived-objects.html)
 the object(s) in question before using rclone.
 
+Note that rclone only speaks the S3 API it does not speak the Glacier
+Vault API, so rclone cannot directly access Glacier Vaults.
+
 <!--- autogenerated options start - DO NOT EDIT, instead edit fs.RegInfo in backend/s3/s3.go then run make backenddocs -->
 ### Standard Options