[Docs] Add a warning about namespace info for HVD

[yhyakuna]

Description

🧵 Slack thread

It was reported that the given example API call does not work due to missing X-Vault-Namespace header when you're working with HCP Vault Dedicated.

Although it was reported specifically with AppRole auth API doc, it is relevant to all API calls --> I cannot just update the AppRole API doc.

So, this PR adds a callout note to the API doc landing page.

TODO only if you're a HashiCorp employee

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• [ ] ENT Breakage: If this PR either 1) removes a public function OR 2) changes the signature of a public function, even if that change is in a CE file, double check that applying the patch for this PR to the ENT repo and running tests doesn't break any tests. Sometimes ENT only tests rely on public functions in CE files.
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