usb_modeswitch: more descriptive help text

Provides a fuller description of the USB_ModeSwitch functionality for
the benefit of the otherwise clueless.

[Thomas: minor formatting and wording fixes.]

Signed-off-by: Jonathan Ben-Avraham <yba@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>