Adding docstring for new get_device method.

src/functor.jl

@@ -552,7 +552,7 @@ julia> Flux.supported_devices()
 supported_devices() = GPU_BACKENDS
 
 """
-    Flux.get_device(; verbose=false)::AbstractDevice
+    Flux.get_device(; verbose=false)::Flux.AbstractDevice
 
 Returns a `device` object for the most appropriate backend for the current Julia session. 
 
@@ -656,6 +656,40 @@ function get_device(; verbose=false)::AbstractDevice
     end
 end
 
+"""
+    Flux.get_device(backend::String, ordinal::Int = 0)::Flux.AbstractDevice
+
+Get a device object for a backend specified by the string `backend` and `ordinal`. The currently supported values
+of `backend` are `"CUDA"`, `"AMD"` and `"CPU"`.
+
+# Examples
+
+```julia-repl
+julia> using Flux, CUDA;
+
+julia> CUDA.devices()
+CUDA.DeviceIterator() for 3 devices:
+0. GeForce RTX 2080 Ti
+1. GeForce RTX 2080 Ti
+2. TITAN X (Pascal)
+
+julia> device0 = Flux.get_device("CUDA", 0)
+(::Flux.FluxCUDADevice) (generic function with 1 method)
+
+julia> device0.deviceID
+CuDevice(0): GeForce RTX 2080 Ti
+
+julia> device1 = Flux.get_device("CUDA", 1)
+(::Flux.FluxCUDADevice) (generic function with 1 method)
+
+julia> device1.deviceID
+CuDevice(1): GeForce RTX 2080 Ti
+
+julia> cpu_device = Flux.get_device("CPU")
+(::Flux.FluxCPUDevice) (generic function with 1 method)
+
+```
+"""
 function get_device(backend::String, ordinal::Int = 0)
     if backend == "CPU"
         return FluxCPUDevice()