The commit e0546fd8 ("usbip: tools: Start using VUDC backend in
usbip tools") implemented device mode for user space tools, however the
corresponding options are not documented in man page.

This commit documents the options and provides examples on device mode.
Also the command `usbip port` is documented.
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Hongren Zheng <i@zenithal.me>
Link: https://lore.kernel.org/r/YFrdyKKx1nx8bktm@SunSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>