docs/qdev-device-use.txt: Add USB U2F key to the QDEV devices examples

Signed-off-by: César Belley <cesar.belley@lse.epita.fr>
Message-id: 20200826114209.28821-10-cesar.belley@lse.epita.fr
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

docs/qdev-device-use.txt

@@ -325,6 +325,7 @@ The new way is -device DEVNAME,DEV-OPTS...  Details depend on DRIVER:
 * mouse           -device usb-mouse
 * tablet          -device usb-tablet
 * wacom-tablet    -device usb-wacom-tablet
+* u2f             -device u2f-{emulated,passthru}
 * braille         See "Character Devices"
 
 === Watchdog Devices ===