Improve embind docs

A bunch of embind docs still have TODOs in them, like the main [bind.h](https://emscripten.org/docs/api_reference/bind.h.html) docs say they are under construction, and the [val.h](https://emscripten.org/docs/api_reference/val.h.html) docs have `HamishW` (original docs author) TODOs and notes.

If someone has time it would be good to fill those out.

cc @chadaustin @brion @surma @RReverser