Add note about using GPU inference for ResNet (#2607)

* Add note about using GPU inference for ResNet

docs/Unity-Inference-Engine.md

 **BroadcastHub** of the Academy.
 Select the **Inference Device** : CPU or GPU you want to use for Inference.
 
-_Note: For most of the models generated with the ML-Agents toolkit, CPU will be faster than GPU. Only use GPU if you have a large number of agents using visual observations._
+**Note:** For most of the models generated with the ML-Agents toolkit, CPU will be faster than GPU.
+You should use the GPU only if you use the 
+ResNet visual encoder or have a large number of agents with visual observations.