add comment to function description

Signed-off-by: Jack <jack.wgm@gmail.com>
add example multi_download.cpp

Signed-off-by: Jack <jack.wgm@gmail.com>
add a open overload

Signed-off-by: Jack <jack.wgm@gmail.com>
fixed calc range on timeout

Signed-off-by: Jack <jack.wgm@gmail.com>