Add bmad details and nvptx compilation instrs.

[JackAKirk]

I've added the bmad (nvptx backend only) feature description that has made use of precision::b1 in the implementation here: https://github.com/intel/llvm/pull/5363

I can also add some other nvidia specific information as requested.

NOTE: this is intended to be merged into https://github.com/intel/llvm/pull/6157, not the intel/sycl branch.