Document a way to (effectively) disable decoder stream memory limit

`liblzma` has [this](https://github.com/kobolabs/liblzma/blob/87b7682c/src/liblzma/api/lzma/container.h#L328-L329) in its docs:

```
* \param memlimit Memory usage limit as bytes. Use UINT64_MAX
* to effectively disable the limiter.
```

This should be documented in the Rust API.

Or, alternatively, replace `limit: u64` with `limit: Option`, in the API and do `let limit = limit.or(u64::MAX)` on the Rust side.