New config MRs should document allowed values

What does this MR do?

This updates our checklist for new merge requests.

When we add a configuration to Omnibus GitLab, sometimes those values are rendered in a template. Depending on the value, it may need special handling, quotations, escapes, or other features.

This checklist items ensures that the author and reviewer consider these when adding a configuration option.

Related issues

This relates to the fix in !5935 (merged), where the expectation at first was an integer. The actual expected value is an integer string and thus needed quotes in the template.

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened