The earlier version had three deficits:
1. It allowed to set the stream to RGB although this is not allowed when
the profile is 0 or 2.
2. If it set the stream to RGB, then it did not automatically set the
range to full range; the result was that one got a warning every time a
frame with color_config element was processed if the frame originally
had TV range and the user didn't explicitly choose PC range. Now one
gets only one warning in such a situation.
3. Intra-only frames in profile 0 are automatically BT.601, but if the
user wished another color space, he was not informed about his wishes
being unfulfillable.

The commit also improves the documentation about this.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>