USB: update intro of documentation

It does no good to mention The 2.4 kernel series and neglect
USB 3.x and XHCI. Also with type C and micro/mini USB we better
not talk about the shape of connectors.

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>