VideoDVCamCaptureDemo: Clarify it doesn't work on Windows or OSX.

Windows doesn't work because video capture in general doesn't
work with GStreamer-1 due to lack of capture plugins in the
official upstream 1.2.4 distro.

OSX doesn't work because Apple. Neither qtkitvideosrc, nor
avfvideosrc were able to get any working video feed from the
camera. Neither did gst commandline or other open source apps.
Only Apples proprietary Facetime app had some marginal success,
so maybe this is a limitation of AVFoundation? Who knows? Who cares..

Linux works with v4l2loopback, so improve setup instructions a bit.