correct mmap parameter type case, add comments
correct mmap parameter type case, add comments

jibsen/imgmin,micmro/imgmin,pornel/imgmin,rflynn/imgmin,micmro/imgmin,jibsen/imgmin,pornel/imgmin,rflynn/imgmin,pornel/imgmin,rflynn/imgmin,pornel/imgmin,micmro/imgmin,micmro/imgmin,jibsen/imgmin,rflynn/imgmin,jibsen/imgmin