Update username template description for AWS (hashicorp#19690)

Update username template description for AWS by calling out what DisplayName and PolicyName actually are placeholders for

website/content/api-docs/secret/aws.mdx

@@ -64,7 +64,9 @@ valid AWS credentials with proper permissions.
 
  To ensure generated usernames are within length limits for both STS/IAM, the template must adequately handle
  both conditional cases (see [Conditional Templates](https://pkg.go.dev/text/template)). As an example, if no template
- is provided the field defaults to the template:
+ is provided the field defaults to the template below. It is to be noted that, DisplayName is the name of the vault 
+ authenticated user running the AWS credential generation and PolicyName is the name of the Role for which the 
+ credential is being generated for:
 
  ```
  {{ if (eq .Type "STS") }}