docs: usb: Add documentation for the UVC Gadget

The UVC Gadget function has become quite complex, but documentation
for it is fairly sparse. Add some more detailed documentation to
improve the situation.

Signed-off-by: Daniel Scally <dan.scally@ideasonboard.com>
Link: https://lore.kernel.org/r/20230308165213.139315-1-dan.scally@ideasonboard.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>