avformat/internal: Improve documentation of ff_packet_list_get

avformat/internal: Improve documentation of ff_packet_list_get

The documentation of ff_packet_list_get currently didn't match the
actual usage:
1. It said that the destination packet is supposed to be initialized.
But this makes no sense given that it will be overwritten completely and
flacenc, mp3enc and ttaenc ignored this.
2. ff_packet_list_get returns an int, although it can't fail in case the
packet list is not empty (for which there is an assert). Again, several
callers didn't check for any return value.
In both cases, the documentation has been adapted to match actual usage.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>