Add API reference to docs

The docs say:
> WARNING: do not use any of the private functionality of the terminal or other non-public parts of the API as these can change in the future.

However, to me it's not really clear everything that is part of the public API. For example, there are functions that take a Terminal object, but it's not immediately obvious what fields of a Terminal can be accessed.
From briefly glancing at the code (to figure out what fields Terminals have) it's already pretty well commented. I don't mind throwing up a PR for this myself, I was just more curious of where you'd think it would be best to put this documentation (ideally so that it's accessible from `:h toggleterm.txt`), as well as determining which parts of the API are intended to be public.