[BUG] Doc: what is a role?

[daks]

Your setup

Formula commit hash / release tag

master

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not relevant

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not relevant

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Bug details

Describe the bug

I was reading doc to better understand map.jinja/TOFS override for formulas. The docs/map.jinja.rst talks about "role" but without explaining what it is and how it works. I think it make this part of the doc cryptic without some previous knowledge.

[baby-gnu]

I agree that role is not straightforward since it's a pure user convention.

I'll propose a little introduction paragraph to explain the idea behind the roles.