explain how -output-curl-string works in comments to avoid confusion

[raskchanky]

Description

Resolves #25213

There is some confusion in the code around how -output-curl-string actually works, so I attempted to explain it in some comments.

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.

[raskchanky]

Given that this doesn't functionally change anything about how Vault works, I don't think it requires a changelog, milestone, or backport.

[github-actions]

CI Results:
All required Go tests succeeded but failures were detected ⚠️
Failures:

Test Type	Package	Test	Logs
race	vault	TestExternalPluginInContainer_MountAndUnmount	view test results
race	vault	TestExternalPluginInContainer_MountAndUnmount/rootful_docker_runtimes	view test results

[github-actions]

Build Results:
All builds succeeded! ✅

[VioletHynes]

This is really awesome. Reading through these, it's immediately obvious the value they provide. Thanks for this! :D