Made it clear a Tween will not update _ properties. Fix #5747

src/tweens/TweenManager.js

@@ -206,6 +206,8 @@ var TweenManager = new Class({
     /**
      * Create a Tween and add it to the active Tween list.
      *
+     * Please note that a Tween will not manipulate any property that begins with an underscore.
+     *
      * @method Phaser.Tweens.TweenManager#add
      * @since 3.0.0
      *

src/tweens/tween/Tween.js

@@ -18,6 +18,8 @@ var MATH_CONST = require('../../math/const');
  * on a duration and type of ease. They are rarely instantiated directly and instead should be
  * created via the TweenManager.
  *
+ * Please note that a Tween will not manipulate any property that begins with an underscore.
+ *
  * @class Tween
  * @memberof Phaser.Tweens
  * @extends Phaser.Events.EventEmitter