Update Feature-Monitor.md to add missing information (#1091)

* Update Feature-Monitor.md

Adds details on 1) how to activate the monitor and 2) how to be able to use a target transform to display the information on.

* Update Feature-Monitor.md

Updated with suggestions from reviewer.

docs/Feature-Monitor.md

 
 The monitor allows visualizing information related to the agents or training process within a Unity scene. 
 
-You can track many different things both related and unrelated to the agents themselves. To use the Monitor, call the Log function anywhere in your code :
+You can track many different things both related and unrelated to the agents themselves. By default, the Monitor is only active in the *inference* phase, so not during training. To change this behaviour, you can activate or deactivate it by calling `SetActive(boolean)`. For example to also show the monitor during training, you can call it in the `InitializeAcademy()` method of your `Academy`:
+
+```csharp
+using MLAgents;
+
+public class YourAcademy : Academy { 
+    public override void InitializeAcademy()
+    {
+        Monitor.SetActive(true);
+    }
+}
+```
+
+To add values to monitor, call the `Log` function anywhere in your code :
 
 ```csharp
 Monitor.Log(key, value, target)
    * *`float[]`* - The Monitor Log call can take an additional argument called `displayType` that can be either `INDEPENDENT` (default) or `PROPORTIONAL` :
    		* *`INDEPENDENT`* is used to display multiple independent floats as a histogram. The histogram will be a sequence of vertical sliders.
    		* *`PROPORTION`* is used to see the proportions between numbers. For each float in values, a rectangle of width of value divided by the sum of all values will be show. It is best for visualizing values that sum to 1.
- * *`target`* is the transform to which you want to attach information. If the transform is `null` the information will be attached to the global monitor.
+ * *`target`* is the transform to which you want to attach information. If the transform is `null` the information will be attached to the global monitor. 
+   * **NB:** When adding a target transform that is not the global monitor, make sure you have your main camera object tagged as `MainCamera` via the inspector. This is needed to properly display the text onto the screen.