[Documentation] Specify how to use the GraphScope (#453)

docs/Learning-Environment-Design-External-Internal-Brains.md

 
 Only change the following Internal Brain properties if you have created your own TensorFlow model and are not using an ML-Agents model:
 
-   *  `Graph Scope` : If you set a scope while training your TensorFlow model, all your placeholder name will have a prefix. You must specify that prefix here.
+   *  `Graph Scope` : If you set a scope while training your TensorFlow model, all your placeholder name will have a prefix. You must specify that prefix here. Note that if more than one Brain were set to external during training, you must give a `Graph Scope` to the internal Brain corresponding to the name of the Brain GameObject.
    *  `Batch Size Node Name` : If the batch size is one of the inputs of your graph, you must specify the name if the placeholder here. The brain will make the batch size equal to the number of agents connected to the brain automatically.
    *  `State Node Name` : If your graph uses the state as an input, you must specify the name of the placeholder here.
    *  `Recurrent Input Node Name` : If your graph uses a recurrent input / memory as input and outputs new recurrent input / memory, you must specify the name if the input placeholder here.