Add where to set uefi variables

[kprosise]

Follow up to Slack discussion, the file the variables UEFI_SIGN_ENABLE and UEFI_SIGN_KEYDIR are set in was mentioned, as well as an admonition mentioning the case where this is not required.

QA steps: ran build and checked rendered HTML, ran linter. No issues found.

No related ticket, 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

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

Let me know if I need to change anything, such as the file, or any unclear wording in the admonition.

[doanac]

Docs for 7eb2537 are browsable at: https://ci.foundries.io/projects/fio-docs/builds/2780/docs/artifacts/html/index.html

[vanmaegima]

lgtm