RISC-V: Document the ISA string parsing rules for ACPI

We've had a ton of issues around the ISA string parsing rules elsewhere
in RISC-V, so let's at least be clear about what the rules are so we can
try and avoid more issues.

Link: https://lore.kernel.org/r/CAK9=C2Vy-4V9kgnga98tiC3TeHkR2LFPakyBbS8s_h3_Z=ieyQ@mail.gmail.com/
Link: https://lore.kernel.org/r/20230629031705.15575-1-palmer@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>