RISC-V: hwprobe: Document unaligned vector perf key

Document key for reporting the speed of unaligned vector accesses.
The descriptions are the same as the scalar equivalent values.

Signed-off-by: Jesse Taube <jesse@rivosinc.com>
Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>
Link: https://lore.kernel.org/r/20241017-jesse_unaligned_vector-v10-6-5b33500160f8@rivosinc.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>