This is included in the ISA manual, but it's pretty common for bits of
the ISA manual that are actually ABI to change.  So let's document it
explicitly.
Reviewed-by: Björn Töpel <bjorn@rivosinc.com>
Link: https://lore.kernel.org/r/20230619190142.26498-1-palmer@rivosinc.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>