📝 Docs: Clarify expectations of maintainers

[JoshuaKGoldberg]

Documentation Request Checklist

• I have read and agree to Mocha's Code of Conduct and Contributing Guidelines
• I have searched for related issues and issues with the faq label, but none matched my issue.
• I want to provide a PR to resolve this

Overview

Surfacing internal discussions: MAINTAINERS.md has very loose verbiage around:

mocha/MAINTAINERS.md

Lines 93 to 95 in 185ae1e

	Furthermore, a maintainer is a contributor who **contributes regularly**, or expresses a _desire to do so._ That could be every day--but it might be once a week, or even once a month.
	Your boss doesn't work here; contribute as often as you wish.
	We are all people with Real Lives, and for many of us, contributing to OSS is just a hobby!

What happens if someone doesn't contribute for more than a month? Months at a time? With or without warning? We'd like to have more clarity in the expectations.

Additional Info

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