No documentation in rules.bzl

Readme says:

> See in-code documentation in [rules.bzl](https://github.com/bazel-contrib/toolchains_llvm/blob/master/toolchain/rules.bzl) for available attributes to llvm_toolchain.

However, the target of the link has no in-code documentation for available attributes to `llvm_toolchain`.

Read the README reference and inspect toolchain/rules.bzl, which is the linked target. Document the available llvm_toolchain attributes there, then verify that the README link leads to the completed documentation.

Written by the indexing model from the issue text.