added docs for engine.timing.isFixed

src/core/Engine.js

@@ -544,6 +544,16 @@ var Engine = {};
      * @default 0
      */
 
+    /**
+     * A `Boolean` that specifies if the `Engine.run` game loop should use a fixed timestep (otherwise it is variable).
+     * If timing is fixed, then the apparant simulation speed will change depending on the frame rate (but behaviour will be deterministic).
+     * If the timing is variable, then the apparant simulation speed will be constant (approximately, but at the cost of determininism).
+     *
+     * @property timing.isFixed
+     * @type boolean
+     * @default false
+     */
+
     /**
      * A `Number` that specifies the time step between updates in milliseconds.
      * If `engine.timing.isFixed` is set to `true`, then `delta` is fixed.