Add text for HVD STS support

Author: jonathanfrappier

content/vault/v1.20.x/content/docs/auth/aws.mdx

@@ -680,6 +680,11 @@ Policy](#recommended-vault-iam-policy) except it doesn't need any further
 Furthermore, in the master account, Vault must be granted the action `sts:AssumeRole`
 for the IAM Role to be assumed.
 
+If you run Vault Enterprise on the [HashiCorp Cloud Platform
+(HCP)](https://portal.cloud.hashicorp.com/sign-in), [contact
+support](https://support.hashicorp.com/hc/en-us/articles/19951252634387-Vault-AWS-Authentication-Cross-Account-Access-with-STS)
+for the cluster ID needed to enable cross account access.
+
 ### AWS instance metadata timeout
 
 @include 'aws-imds-timeout.mdx'

content/vault/v1.21.x/content/docs/auth/aws.mdx

@@ -680,6 +680,11 @@ Policy](#recommended-vault-iam-policy) except it doesn't need any further
 Furthermore, in the master account, Vault must be granted the action `sts:AssumeRole`
 for the IAM Role to be assumed.
 
+If you run Vault Enterprise on the [HashiCorp Cloud Platform
+(HCP)](https://portal.cloud.hashicorp.com/sign-in), [contact
+support](https://support.hashicorp.com/hc/en-us/articles/19951252634387-Vault-AWS-Authentication-Cross-Account-Access-with-STS)
+for the cluster ID needed to enable cross account access.
+
 ### AWS instance metadata timeout
 
 @include 'aws-imds-timeout.mdx'