docs: riscv: Define behavior of mmap

Define mmap on riscv to not provide an address that uses more bits than
the hint address, if provided.

Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
Link: https://lore.kernel.org/r/20240130-use_mmap_hint_address-v3-3-8a655cfa8bcb@rivosinc.com


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