website: update docs for ami_groups claritifaction [GH-1068]

website/source/docs/builders/amazon-ebs.html.markdown

@@ -88,6 +88,7 @@ each category, the available configuration keys are alphabetized.
 * `ami_groups` (array of strings) - A list of groups that have access
   to launch the resulting AMI(s). By default no groups have permission
   to launch the AMI. `all` will make the AMI publicly accessible.
+  AWS currently doesn't accept any value other than "all".
 
 * `ami_product_codes` (array of strings) - A list of product codes to
   associate with the AMI. By default no product codes are associated with

website/source/docs/builders/amazon-instance.html.markdown

@@ -107,6 +107,7 @@ each category, the available configuration keys are alphabetized.
 * `ami_groups` (array of strings) - A list of groups that have access
   to launch the resulting AMI(s). By default no groups have permission
   to launch the AMI. `all` will make the AMI publicly accessible.
+  AWS currently doesn't accept any value other than "all".
 
 * `ami_product_codes` (array of strings) - A list of product codes to
   associate with the AMI. By default no product codes are associated with