Modified the player brain doc to mention that the default value is 0.

docs/Learning-Environment-Design-Player-Brains.md

 |                               | **Element 0–N**  | The mapping of keys to action values.                                                                                                                                                                                                     |
 |                               | **Key**          | The key on the keyboard.                                                                                                                                                                                                                  |
 |                               | **Branch Index** | The element of the Agent's action vector to set when this key is pressed. The index value cannot exceed the size of the Action Space (minus 1, since it is an array index).                                                               |
-|                               | **Value**        | The value to send to the Agent as its action when the mapped key is pressed. Cannot exceed the max value for the associated branch (minus 1, since it is an array index).                                                                 |
+|                               | **Value**        | The value to send to the Agent as its action when the mapped key is pressed. Cannot exceed the max value for the associated branch (minus 1, since it is an array index). Note that if no key is pressed for that branch, the default action will be 0.                                                                |
 
 For more information about the Unity input system, see
 [Input](https://docs.unity3d.com/ScriptReference/Input.html).