Improve documentation for what's the semantics of the content types metadata

[reyammer]

For magika up to 0.5.1, the content type metadata included things such as "magic" and "vt_type", which ideally we should document. However, we cleaned most of these things up in magika 0.6.0+, so it's unclear whether we have something to improve on this aspect. Opening this issue as a placeholder until we figure that out.