Document scope.name:root-scope-used activation hook

[Arcanemagus]

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Description of the Change

Added to Atom since v1.30.0 in https://github.com/atom/atom/pull/17521, the scope.name:root-scope-used activation hook wasn't currently documented anywhere other than the PR that added it.

This adds it as an example and gives some explanation for the other two hooks as well.

Release Notes

N/A

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Closes #474.

[Arcanemagus]

Hmmm, this is actually an alternative to #474, will try to get a consensus on which should be merged.