gpu docstring

src/functor.jl

@@ -225,8 +225,10 @@ end
 Copies `m` to the current GPU device (using current GPU backend), if one is available.
 If no GPU is available, it does nothing (but prints a warning the first time).
 
-On arrays, this calls CUDA's `cu`, which also changes arrays
-with Float64 elements to Float32 while copying them to the device (same for AMDGPU).
+When the backed is set to "CUDA", when called on arrays it calls `CUDA.cu`, 
+which also changes arrays with Float64 elements to Float32 while copying them to the device. 
+Similar conversions happen for "AMDGPU" and "Metal" backends.
+
 To act on arrays within a struct, the struct type must be marked with [`@functor`](@ref).
 
 Use [`cpu`](@ref) to copy back to ordinary `Array`s.
@@ -235,6 +237,8 @@ See also [`f32`](@ref) and [`f16`](@ref) to change element type only.
 See the [CUDA.jl docs](https://juliagpu.github.io/CUDA.jl/stable/usage/multigpu/) 
 to help identify the current device.
 
+See [`Flux.gpu_backend!`](@ref) for setting the backend.
+
 # Example
 ```julia-repl
 julia> m = Dense(rand(2, 3))  # constructed with Float64 weight matrix