avenc_dnxhd: Undocumented restrictions to width/height/framerate/bitrate settings

Submitted by Sebastian Dröge @slomo

Link to original bug (#574725)

Description

Hi,
ffenc_dnxhd only allows some very restrictive width/height/framerate/bitrate settings, see libavcodec/dnxhddata.c the ff_dnxhd_cid_table.

We should document this and specify this in the caps and tell the ffmpeg people to simply choose the nearest possible bitrate instead or requiring a bitrate the exactly fits with the width/height/framerate combination.