Rename decision frequency to interval (#1697)

UnitySDK/Assets/ML-Agents/Editor/AgentEditor.cs

                 EditorGUILayout.PropertyField(
                     actionsPerDecision,
                     new GUIContent(
-                        "Decision Frequency",
+                        "Decision Interval",
                         "The agent will automatically request a decision every X" +
                         " steps and perform an action at every step."));
                 actionsPerDecision.intValue = Mathf.Max(1, actionsPerDecision.intValue);

docs/Learning-Environment-Create-New.md

     window.
 3. Drag the Brain **RollerBallPlayer** from the Project window to the 
     RollerAgent **Brain** field.
-4. Change **Decision Frequency** from `1` to `10`.
+4. Change **Decision Interval** from `1` to `10`.
 5. Drag the Target GameObject from the Hierarchy window to the RollerAgent
     Target field.
 

docs/Learning-Environment-Design-Agents.md

     * `RequestAction()` Signals that the Agent is requesting an action. The
         action provided to the Agent in this case is the same action that was
         provided the last time it requested a decision.
-* `Decision Frequency` - The number of steps between decision requests. Not used
+* `Decision Interval` - The number of steps between decision requests. Not used
   if `On Demand Decision`, is true.
 
 ## Monitoring Agents