transcode for ByteData instead of only Vector{UInt8}

I noticed that the documentation says, that the signature of `transcode` is
```julia
transcode(codec::Codec, data::Vector{UInt8})::Vector{UInt8}
```

but then for the code we have the signature

```julia
transcode(codec::Codec, data::ByteData)
```

Should maybe maybe the documentation be adjusted?