Add a specific reference to AWS IAM Unique Identifiers (#8209)

* Add specification about AWS IAM Unique Identifiers

We experienced an issue where IAM roles resources were re-provisioned with the same ARNs and no change had been made to our vault role configuration but users lost access with `-method=aws`. It wasn't immediately clear to us how IAM Unique Identifiers where being used to avoid the same situations outlined in the AWS documentation. We eventually concluded that re-provisioning the roles in our auth/aws/auth would fetch the new IAM Unique Identifiers. 

I hope that this small amendment helps people avoid this problem in the future.

website/pages/docs/auth/aws.mdx

@@ -317,7 +317,7 @@ are needed.
   `ec2` auth method. Vault needs to determine which IAM role is attached to the
   instance profile.
 - `iam:GetUser` and `iam:GetRole` are used when using the iam auth method and
-  binding to an IAM user or role principal to determine the unique AWS user ID
+  binding to an IAM user or role principal to determine the [AWS IAM Unique Identifiers](https://docs.aws.amazon.com/IAM/latest/UserGuide/reference_identifiers.html#identifiers-unique-ids)
   or when using a wildcard on the bound ARN to resolve the full ARN of the user
   or role.
 - The `sts:AssumeRole` stanza is necessary when you are using [Cross Account