C API: document error handling

[ehmry]

The documentation on the C API should also include error handling because there are too many ways to imagine how it works. If a procedure returns nix_err I don't know if that means without a context it will just throw an exception, and if a context is set I don't know if the context will carry the same error as the one returned.

Perhaps it would be useful to handler errors at a context using a callback? This would allow bindings to throw errors as their own native exceptions.

[roberth]

It looks like the main page is broken

• https://hydra.nixos.org/build/264899401/download/1/html/index.html

We do have this, but we can't really expect anyone to find it without a main page.