Document how to get from TrackedFoo to Foo

[oxinabox]

See each this conversation.

<@JonasIsensee>
Hey there,
I just started playing with Flux and ML in general for the first time.

How can I plot predictions?
scatter(y, predict.(x))
gives me the following
MethodError: no method matching Float64(::Flux.Tracker.TrackedReal{Float64}

<@JonasIsensee>
This seems to work
yhat = map(x -> x[1].data, predict.(x))

I can never remember how to do it.
And I always endup looking around the code base for:

Flux.jl/src/tracker/scalar.jl

Line 8 in b93d476

data(x::TrackedReal) = x.data

related:

Is data(xs::AbstractArray{<:TrackedReal}) = data.(xs) defined?

Possibly it should be, if not
so that
AbstractArray{<:TrackedReal} and TrackedArray act the same

[MikeInnes]

We can mention this in the docs if we aren't already. Calling Tracker.data(x) seems like it's about as simple as this is going to get though.

The AbstractArray{<:TrackedReal} thing is tricky, especially because you don't want AbstractArray{Any} to behave differently. But I don't think it'd hurt to just always broadcast over any abstractarray.