Be more explicit about script/extension

[n8willis]

The current docs are somewhat informal about what characters / code-points are expected (e.g., punctuation from Devanagari in other Indic2 scripts). The special-purpose stuff like ZW[N]J obviously benefits from inline explanations, but it would be good to be more strict about the other stuff, i.e. https://github.com/roozbehp/unicode-data/tree/master/exemplars

Inspired by https://twitter.com/TiroTypeworks/status/1492964039818821634 / surrounding thread.