improved docs, fixed dirty on group destroy (#4293)

src/mixins/collection.mixin.js

@@ -10,6 +10,9 @@ fabric.Collection = {
    * (if `renderOnAddRemove` is not `false`).
    * in case of Group no changes to bounding box are made.
    * Objects should be instances of (or inherit from) fabric.Object
+   * Use of this function is highly discouraged for groups.
+   * you can add a bunch of objects with the add method but then you NEED
+   * to run a addWithUpdate call for the Group class or position/bbox will be wrong.
    * @param {...fabric.Object} object Zero or more fabric instances
    * @return {Self} thisArg
    * @chainable
@@ -28,6 +31,9 @@ fabric.Collection = {
   /**
    * Inserts an object into collection at specified index, then renders canvas (if `renderOnAddRemove` is not `false`)
    * An object should be an instance of (or inherit from) fabric.Object
+   * Use of this function is highly discouraged for groups.
+   * you can add a bunch of objects with the insertAt method but then you NEED
+   * to run a addWithUpdate call for the Group class or position/bbox will be wrong.
    * @param {Object} object Object to insert
    * @param {Number} index Index to insert object at
    * @param {Boolean} nonSplicing When `true`, no splicing (shifting) of objects occurs

src/shapes/group.class.js

@@ -393,6 +393,11 @@
      * @chainable
      */
     destroy: function() {
+      // when group is destroyed objects needs to get a repaint to be eventually
+      // displayed on canvas.
+      this._objects.forEach(function(object) {
+        object.set('dirty', true);
+      });
       return this._restoreObjectsState();
     },