* 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.
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