Update namespaces.mdx add @include 'alerts/enterprise-only.mdx'

[owenzorrin]

Add "@include 'alerts/enterprise-only.mdx'" since namespace is an enterprise feature

Description

Received a request state that Vault namespace API doc doesnt have the enterprise feature reminder

Add "@include 'alerts/enterprise-only.mdx'"

TODO only if you're a HashiCorp employee

• Backport Labels: If this PR is in the ENT repo and needs to be backported, backport
  to N, N-1, and N-2, using the backport/ent/x.x.x+ent labels. If this PR is in the CE repo, you should only backport to N, using the backport/x.x.x label, not the enterprise labels.
  • If this fixes a critical security vulnerability or severity 1 bug, it will also need to be backported to the current LTS versions of Vault. To ensure this, use all available enterprise labels.
• 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.
• Jira: If this change has an associated Jira, it's referenced either
  in the PR description, commit message, or branch name.
• RFC: If this change has an associated RFC, please link it in the description.
• ENT PR: If this change has an associated ENT PR, please link it in the
  description. Also, make sure the changelog is in this PR, not in your ENT PR.

[github-actions]

CI Results:
All Go tests succeeded! ✅

[github-actions]

Build Results:
All builds succeeded! ✅

[owenzorrin]

Created pull due to customer request that namespace is Vault enterprise feature

[owenzorrin]

@schavis please have a review of this pull request, thank you! Let me know if there is any question.

[schavis]

@schavis please have a review of this pull request, thank you! Let me know if there is any question.

Nope, looks good.