This commit updates the documentation of av_read_frame() to match its
actual behaviour in several ways:

1. On success, av_read_frame() always returns refcounted packets.
2. It can handle uninitialized packets.
3. On error, it always returns blank packets.

This will allow callers to not initialize or unref unnecessarily.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>