Hi

Rather than continue the RFC thread for this, I decided to submit a
patch that begins the process of documenting metadata keys.

There's quite a lot of keys used in the codebase, and this patch does
not cover all of them. But it should give the general idea. I follow a
similar approach to that used by ffmpeg -codecs to try and keep the
number of columns sane.

One issue I discovered is that pod2man does not seem to support
@multitable very well, and @headitem causes it to fail outright. Not
sure what to do about that. This issue exists with other @multitable in
the documentation. See "man doc/ffmpeg-all.1" and search for "Audio
Codecs"

Anyway, thoughts on this?

/Tomas