Add admonition for credential helper on macOS

[kprosise]

Used the important admonition with how to locate the line to comment out to get the credential helper to work on macOS. Left out other troubleshooting steps, which can be added if warranted.

QA steps: checked rendered output in browser, ran linter.

No related tasks, quick fix.

PR Template and Checklist

Please complete as much as possible to speed up the reviewing process.

Readiness and adding reviewers as appropriate is required.

All PRs should be reviewed by a technical writer/documentation team and a peer.
If effecting customers—which is a majority of content changes—a member of Customer Success must also review.

Readiness

• Merge (pending reviews)
• Merge after date or event
• Draft

Overview

Why merge this PR? What does it solve?

Checklist

• Run spelling and grammar check, preferably with linter.
• Avoid changing any header associated with a link/reference.
• Step through instructions (or ask someone to do so).
• Review for wordiness
• Match tone and style of page/section.
• Run make linkcheck.
• View HTML in a browser to check rendering.
• Use semantic newlines.
• follow best practices for commits.
  • Descriptive title written in the imperative.
  • Include brief overview of QA steps taken.
  • Mention any related issues numbers.
  • End message with sign off/DCO line (-s, --signoff).
  • Sign commit with your gpg key (-S, --gpg-sign).
  • Squash commits if needed.
• Request PR review by a technical writer and at least one peer.

Comments

Any thing else that a maintainer/reviewer should know.
This could include potential issues, rational for approach, etc.

[doanac]

Docs for 1278f82 are browsable at: https://ci.foundries.io/projects/fio-docs/builds/2697/docs/artifacts/html/index.html