Document `r_clang` compiler option

The `r-arrow` recipe uses an `r_clang` compiler on Windows:

https://github.com/conda-forge/r-arrow-feedstock/blob/99eee2298644d6439115a96ca5fd47f23378f353/recipe/meta.yaml#L31

However, the documentation only describes five compilers, which does not include this:

https://github.com/conda-forge/conda-forge.github.io/blob/1d3214c295a46a249434de4fcf48c6b8d747a07f/src/maintainer/knowledge_base.rst?plain=1#L280-L287

This should be documented with guidance on usage.