Documentation for exported APIs

There's no direct documentation of the built-in actors exported APIs. There's only little documentation of any API, but that's been tolerable to date because the set of users is small and generally has high context already.

We should develop some kind of decent quality documentation of the syntax and semantics of APIs callable by other actors or smart contracts. Immediate motivation for this comes from EVM developers, but there's nothing EVM-specific about this documentation (I'd expect EVM library authors could derive much of their documentation from ours, though).

It's not immediately obvious what the best way to do this is.
- Standard Rust doc tooling would include all Rust-visible methods, which is too much
- Maintaining it separately will add some development burden and duplication
- Custom tooling to generate it from our code structures and patterns could be good, but high effort

FYI @arajasek @raulk @ZenGround0