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.

Start by locating the getFileStream API and the Config documentation in this package, then read the source to confirm what the key parameter controls. Document that behavior clearly, including how it differs from CachedNetworkImage's cacheKey, and verify that the public API documentation now explains the parameter.

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