modules/github-runner: Improve description of `url`

We spent a whole afternoon debugging this, because upstream has very
bad software quality and the error messages were incredibly
misleading.

So let’s document it for the sanity of other people.

Btw, I think the implementation of our module is pretty brittle,
especially the part about diffing tokens to check whether they
changed. We should rather just request a new builder registration
every time, it’s not that much overhead, and always set `replace` so
it is idempotent.

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