avfilter: add comments for duplicate line

avfilter: add comments for duplicate line comment about the looks like a duplicate line.
but that is used to reason x is expressed from y Suggested-by: Paul B Mahol
Suggested-by: Michael Niedermayer
Signed-off-by: Steven Liu 

• [DH] libavfilter/vf_crop.c
• [DH] libavfilter/vf_drawtext.c
• [DH] libavfilter/vf_overlay.c