Document toolchain builtin function extension

[zygoloid]

The toolchain has an extension for declaring a builtin function:

fn F() = "no_op";

This is used pervasively by the toolchain to expose built-in primitive functionality to the prelude.

We should document this extension, either in the language design or at least in the toolchain documentation, and also consider a short proposal introducing it and comparing it to other options (such as __builtin_function or recognizing certain library names as being builtins).