doc/encoders: replace @xref with @ref command

@xref is ignored by texi2pod.
avformat/utils: make "first_dts not matching first dts in the queue" message more informative

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
ffmpeg_filter: Fix non jpeg yuv in jpeg support

This is a regression, did not bisect so dont know what caused it but
likely some changes to the command line handling code.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>