API documentation is highly incomplete

Consider the `cxx_toolchain()` API (https://buck2.build/docs/api/rules/#cxx_toolchain).

There are only 7 documented arguments, and a whopping 99 total arguments. A similar ratio exists for many other APIs. Note that the `cxx_toolchain()` API has no _self_ documentation either (what is this API, why do you use it, example invocation, etc) and that also is pervasive among the API documentation.