doc/general.texi: Adjust the notes on AviSynth

FFmpeg provides local copies of these headers in compat/avisynth/,
and there is no restriction against using 2.5.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/movenc: rescale when copying duration to timecode track

Fixes Ticket3453

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