avcodec/bsf: mention that av_bsf_send_packet() returning EAGAIN is not an error

avcodec/bsf: mention that av_bsf_send_packet() returning EAGAIN is not an error

EAGAIN is returned when input is provided but can't be consumed. The filtering
process is unaffected in this case, and the function will be able to consume
new input after retrieving filtered packets with av_bsf_receive_packet().

Remove the line about empty packets never failing added in
41b05b849f215b03eeb9e3608571ba47de64182a while at it. Even if it's currently
the case, it unnecessarily constrains the API and could be changed in the future
in case it needs to be extended.
The user should always check for errors and never expect a call to never fail.

Reviewed-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>