lavd/avfoundation: Introduce device alias 'none' to allow the user to record only audio or video.

Changes the selection of a default device to none instead of the system default device.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Changelog: Mention AVFoundation screen capturing.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
doc/indevs: Rework and update documentation of AVFoundation device.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>