Provide info about default qualitiy for each codec

[Ulf Zibis]

Summary of the bug:
The default value for option -q is missing, at least for encoder=libx264.
How to reproduce:

• search on https://ffmpeg.org/ffmpeg-codecs.html
• additionally try:

  % ffmpeg -h encoder=libx264
  ffmpeg version 4.1.3
  built on gcc 6.3.0 (Debian 6.3.0-18+deb9u1) 20170516
  

From the output of using this encoder, I can see, it seems to be 28.0, but this is nowhere documented.

IMHO on https://ffmpeg.org/ffmpeg-codecs.html it should be possible to easily find for each codec:

1. the default quality
2. which option to use to change it (here -q)

Additionally ffmpeg -h encoder=blafoo should give this info.