docs: font per-call preload scope and the declarations font-family override (#99167)
Derived from the conversation over at:
https://github.com/vercel/next.js/discussions/99025
I think `preload` needs that additional caveat.
Document the declaration of a font-family. Note that the pattern
discussed on that thread is not something we should document, at least I
don't think so, I am just expanding the docs on this area.