doc: update ffmpeg -ar and -ac documentation to reflect reality.

doc/ffmpeg.texi

@@ -551,19 +551,19 @@ The timestamps must be specified in ascending order.
 @item -aframes @var{number}
 Set the number of audio frames to record.
 @item -ar @var{freq}
-Set the audio sampling frequency. For input streams it is set by
-default to 44100 Hz, for output streams it is set by default to the
-frequency of the input stream. If the input file has audio streams
-with different frequencies, the behaviour is undefined.
+Set the audio sampling frequency. For output streams it is set by
+default to the frequency of the corresponding input stream. For input
+streams this option only makes sense for audio grabbing devices and raw
+demuxers and is mapped to the corresponding demuxer options.
 @item -ab @var{bitrate}
 Set the audio bitrate in bit/s (default = 64k).
 @item -aq @var{q}
 Set the audio quality (codec-specific, VBR).
 @item -ac @var{channels}
-Set the number of audio channels. For input streams it is set by
-default to 1, for output streams it is set by default to the same
-number of audio channels in input. If the input file has audio streams
-with different channel count, the behaviour is undefined.
+Set the number of audio channels. For output streams it is set by
+default to the number of input audio channels. For input streams
+this option only makes sense for audio grabbing devices and raw demuxers
+and is mapped to the corresponding demuxer options.
 @item -an
 Disable audio recording.
 @item -acodec @var{codec}