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.

まず、このパッケージ内で getFileStream API と Config のドキュメントを見つけ、ソースを読んで key パラメーターが制御する内容を確認します。その動作を明確にドキュメント化し、CachedNetworkImage の cacheKey との違いも含め、公開 API のドキュメントでこのパラメーターが説明されるようになっていることを確認します。

索引モデルが issue の本文から書いたものです。