RISC-V: Add some comments about the shadow and overflow stacks

It took me a while to page all this back in when trying to review the
recent spin_shadow_stack, so I figured I'd just write up some comments.

Reviewed-by: Guo Ren <guoren@kernel.org>
Reviewed-by: Jisheng Zhang <jszhang@kernel.org>
Link: https://lore.kernel.org/r/20221130023515.20217-2-palmer@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>