Document getFileStream.key parameter

## 🏗 Enhancement Proposal

Some methods such as getFileStream have a `key` parameter. This is very useful in some cases, but is not documented. I suggest adding a little bit of documentation about that parameter.

I'm creating this issue because I myself had to look at the source code to make sure that the parameter does what I expect from it. Specifically, I was looking at the `cacheKey` parameter of CachedNetworkImage in https://github.com/Baseflow/flutter_cached_network_image. This is named unclearly, because the same name is used as a parameter in `Config` in this package, with a completely unrelated meaning. Of course documentation should also be added in that package.

### Pitch

More complete documentation makes this package easier to use correctly.

Commencez par localiser l’API getFileStream et la documentation de Config dans ce package, puis lisez le code source pour confirmer ce que contrôle le paramètre key. Documentez clairement ce comportement, notamment en quoi il diffère de cacheKey de CachedNetworkImage, et vérifiez que la documentation de l’API publique explique désormais ce paramètre.

Rédigé par le modèle d'indexation à partir du texte de l'issue.