docs: add VPC-related note to auth docs

Signed-off-by: Phil Adams <phil_adams@us.ibm.com>

Authentication.md

@@ -453,7 +453,8 @@ The IAM access token is added to each outbound request in the `Authorization` he
 - IAMProfileID: (optional) the id of the linked trusted IAM profile to be used when obtaining the IAM access token.
 
 - URL: (optional) The base endpoint URL of the VPC Instance Metadata Service.  
-The default value of this property is `http://169.254.169.254`, and should not need to be specified in normal situations.
+The default value of this property is `http://169.254.169.254`.  However, if the VPC Instance Metadata Service is configured
+with the HTTP Secure Protocol setting (`https`), then you should configure this property to be `https://api.metadata.cloud.ibm.com`.
 
 - Client: (optional) The `http.Client` object used to interact with the VPC Instance Metadata Service.
 If not specified by the user, a suitable default Client will be constructed.