Document/Decide on adapt(Backend, ...) API

[vchuravy]

In #364 I added adapt(backend, x) as a natural extension for adapt, instead of having to use something like adapt(default_array_type(backend), x) which we don't currently have.

This was inspired by #353 and #297

As noted by @maleadt I didn't document it and it is a rather unofficial API that has lead users astray JuliaGPU/CUDA.jl#2459 (comment)

Ideally it would simply be adapt(::CUDABackend, x) = adapt(CuArray, x). I am not sure why I didn't use that definition before.