RISC-V: mm: Document mmap changes

The behavior of mmap is modified with this patch series, so explain the
changes to the mmap hint address behavior.

Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20230809232218.849726-5-charlie@rivosinc.com


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