Discarding all frames is universally supported as it happens in fftools
whereas selective discarding relies on the demuxer.
rtp stream. (Requires at least one of the output formats to be rtp).
@item -discard (@emph{input})
-Allows discarding specific streams or frames of streams at the demuxer.
-Not all demuxers support this.
+Allows discarding specific streams or frames from streams.
+Any input stream can be fully discarded, using value @code{all} whereas
+selective discarding of frames from a stream occurs at the demuxer
+and is not supported by all demuxers.
@table @option
@item none