Add note about vault kv get to Proxy docs

[VioletHynes]

Description

This has tripped up something like five people in as many months that I'm aware of. As a result, I feel like it's worth a call-out in the docs as it's such a common point of confusion. This also adds something to the docs to clarify that secrets can be accessible even when Vault is unavailable, which is something we didn't have explicitly before.

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.

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[ccapurso]

Looks good, thanks for adding that! My only question is whether or not we should backport this so that it is live for existing doc versions?

[VioletHynes]

Good call-out about the backport. I'll backport it too!