Our reference docs don't currently include members inherited through the
`extends` keyword. These extended items are barely referenced at all -
you have to find them in the signature.
This diff adds a clearer note to the docs saying which type has been
extended, and if possible brings the extended items through onto the
current documentation page (with a note saying where they're from)
### Change Type
- [x] `docs` — Changes to the documentation, examples, or templates.
- [x] `improvement` — Improving existing features
c4b9ea30f4