docs: improve third-party reporter docs

[mark-wiemer]

PR Checklist

• Addresses an existing open issue: fixes 📝 Docs: No documentation on third party reporters example #3104
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

• Document the Runner.constants value used by reporters
• Mention the "missing" documentation in Runnable class
  • Runnable.prototype.xyz calls are not published to the API doc site, leading to confusion about which properties are included in a Runnable object, like a Test or a Hook.

[mark-wiemer]

@JoshuaKGoldberg this is ready for review, please let me know what I'm doing wrong with these darn compliance checks, I've read the verbose logs and I'm still confused!

[JoshuaKGoldberg]

From https://github.com/mochajs/mocha/actions/runs/12599907071/job/35117787163?pr=5285:

Warning: PR Body did not match required format

Heh. Not exactly informative. I think this would be fixed by mtfoley/pr-compliance-action#368 -> mtfoley/pr-compliance-action#374.

Anyway, it's that this doesn't "fix" an existing issue - just "works on". I edited the body now that the other PR is resolved. ✅

[JoshuaKGoldberg]

LGTM! Just a couple of question/touchup points?

[JoshuaKGoldberg]

Lovely, thanks :)