docs: explain per-route guards on nested routes

[skirtles-code]

While working on #2127, I had originally intended to include a note about per-route guards, but the specific functionality I was trying to reference doesn't appear to be documented, so I left it out.

This PR attempts to address that.

Specifically, when using nested routes, the beforeEnter per-route guard can be placed on either the parent or the child route. There is some subtlety to exactly when those guards will be called, which I believe is intentional and worth documenting.

I've also moved the final paragraph of that section into a tip. The new content about nested routes left that final paragraph feeling like a strange jump to a separate topic, and I think the tip box makes that less confusing.

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