demux_lavf: don't consider EAGAIN as EOF condition

This happens apparently randomly with rtmp:// and after seeks. This
eventually leads to audio decoding returning an EOF status, which
basically disables audio sync. This will lead to audio desync, even if
audio decoding later "recovers" when the demuxer actually returns audio
packets.

Hack-fix this by special-casing EAGAIN.
client API: fix typos in documentation