avcodec: clarify some decoding/encoding API details

Make it clear that there is no timing-dependent behavior. In particular,
there is no state in which both input and output are denied, and where
you have to wait for a while yourself to make progress (apparently some
hardware decoders like to do this).

Avoid wording that makes references to time. It shouldn't be mistaken
for some kind of asynchronous API (like POSIX read() can return EAGAIN
if there is no new input yet). It's a state machine, so try to use
appropriate terms.

Signed-off-by: Diego Biurrun <diego@biurrun.de>

Merges Libav commit 8a60bba0.