Try to explain that `font-path` is specifically for *custom* fonts

[jfly]

I am not the only person who mistakenly thought that /mnt/onboard/fonts is supposed to always exist, and should contain a number of fonts out of the box. See this bit of confusion for why I think this comment would be helpful: https://github.com/baskerville/plato/issues/88#issuecomment-1791947072