V4L/DVB (3653h): Move usb v4l docs into Documentation/video4linux

- Move documentation for usb v4l devices from
  Documentation/usb to Documentation/video4linux.
- Removed trailing whitespace.
- Update Kconfig help text links to reflect the new file locations.

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>