Add text for HVD STS support (#1459)

## Description

<!-- ID for Jira ticket e.g [SPE-1234] -->

:ticket: [SPE-1376]

<!-- Add a brief description of changes here. Include any other
necessary relevant links -->
- Add note for HVD cross account access with AWS auth method and link to
support doc
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selecting the scope and urgency. -->

### Requested review scope:

- [x] Content touched by the PR _only_ (typos, clarifications, tips)
- [ ] Code test (command and code block changes)
- [ ] Flow and language near changes (new/rearranged steps)
- [ ] Review everything (rewrites, major changes)

### Review urgency:

- [ ] ASAP (bug fixes, broken content, imminent releases)
- [ ] 3 days (small changes, easy reviews)
- [ ] 1 week (default)
- [x] Best effort (very non-urgent)

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## All updates:

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I have:

- [ ] Verified that all status checks have passed
- [x] Verified that preview environment has successfully deployed
- [x] Verified appropriate `label` applied (`hcp` + `product name`)
- [x] Added all required reviewers (code owners and external)

[SPE-1234]:
https://hashicorp.atlassian.net/browse/SPE-1234?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ
[SPE-1376]:
https://hashicorp.atlassian.net/browse/SPE-1376?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

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'