adjustaments

It's long but don't really know how to short this description line more ^^"
I've added the long forms of `c:` and `b:v`. Threads has been added because I noticed that if expressed as `0` or not set, only 1 thread is used. (tested on version 3.4.1-1)

pages/common/ffmpeg.md

@@ -26,6 +26,6 @@
 
 `ffmpeg -i {{input_video}}.mkv -codec copy {{output_video}}.mp4`
 
-- Convert MP4 video to VP9 codec. The CRF value can be from 0–63. Lower values mean better quality. -b:v MUST be 0:
+- Convert MP4 video to VP9 codec. For the best quality, use a CRF value (recommended range 15-35) if so, -b:video MUST be 0:
 
-`ffmpeg -i {{input_video}}.mp4 -c:v libvpx-vp9 -crf 30 -b:v 0 {{output_video}}.webm`
+`ffmpeg -i {{input_video}}.mp4 -codec:video libvpx-vp9 -crf 30 -b:video 0 -codec:audio libopus -vbr on -threads {{number_of_threads}} {{output_video}}.webm`