gst-ffmpeg: expand package description to describe libav usage

- Change URL to gst-ffmpeg page at the gstreamer site.
- Note that gst-ffmpeg actually uses libav, not ffmpeg [anymore].
- Note that gst-ffmpeg is using an internal copy of libav, not a
  system copy.

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Acked-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>