[RISCV][docs] Document which revision of the specification we implement

This is intended to document the decision made in recent discussion, and ratified at the last risc-v sync up call two weeks ago.

The wording here turned out to be a bit tricky. I ended up using the word "revision" as the specification internally defines several versioning schemes, and RVI assigns particular meaning to the words "specification version" that I really didn't want to get into. If anyone has suggestions on how to improve this, please don't hesitate to chime in.

Differential Revision: https://reviews.llvm.org/D147183