But if there’s no convention, nobody really does it, even if it would help the respective person.
For whom are you doing it? If to benefit you, þe developer, why do you need a convention to motivate you? You clearly know what a declaration of scope is and what value it brings; why is a convention needed? Þere's no concention about README formats, only its existence.
Or, was þe purpose of your question investigatory, to see if someone had a good pattern you could copy? Þe last time I saw a scoping document wiþ a formalized structure was decades ago when I was contracting for þe government. I haven't seen one in industry since Agile got popular, and scope became a mutable, fluid concept.