docs($rootScope.Scope): clarify $watchGroup "oldValues" argument

This should help to prevent issues such as #8671, #12452, #16004.

Note that the behavior will change in 1.7 (see #15854)

Closes #12643
Closes #16005

Author: Narretz

src/ng/rootScope.js

@@ -449,6 +449,12 @@ function $RootScopeProvider() {
        *   values are examined for changes on every call to `$digest`.
        * - The `listener` is called whenever any expression in the `watchExpressions` array changes.
        *
+       * `$watchGroup` is more performant than watching each expression individually, and should be
+       * used when the listener does not need to know which expression has changed.
+       * If the listener needs to know which expression has changed,
+       * {@link ng.$rootScope.Scope#$watch $watch()} or
+       * {@link ng.$rootScope.Scope#$watchCollection $watchCollection()} should be used.
+       *
        * @param {Array.<string|Function(scope)>} watchExpressions Array of expressions that will be individually
        * watched using {@link ng.$rootScope.Scope#$watch $watch()}
        *
@@ -457,7 +463,34 @@ function $RootScopeProvider() {
        *    The `newValues` array contains the current values of the `watchExpressions`, with the indexes matching
        *    those of `watchExpression`
        *    and the `oldValues` array contains the previous values of the `watchExpressions`, with the indexes matching
-       *    those of `watchExpression`
+       *    those of `watchExpression`.
+       *
+       *    Note that `newValues` and `oldValues` reflect the differences in each **individual**
+       *    expression, and not the difference of the values between each call of the listener.
+       *    That means the difference between `newValues` and `oldValues` cannot be used to determine
+       *    which expression has changed / remained stable:
+       *
+       *    ```js
+       *
+       *    $scope.$watchGroup(['v1', 'v2'], function(newValues, oldValues) {
+       *      console.log(newValues, oldValues);
+       *    });
+       *
+       *    // newValues, oldValues initially
+       *    // [undefined, undefined], [undefined, undefined]
+       *
+       *    $scope.v1 = 'a';
+       *    $scope.v2 = 'a';
+       *
+       *    // ['a', 'a'], [undefined, undefined]
+       *
+       *    $scope.v2 = 'b'
+       *
+       *    // v1 hasn't changed since it became `'a'`, therefore its oldValue is still `undefined`
+       *    // ['a', 'b'], [undefined, 'a']
+       *
+       *    ```
+       *
        *    The `scope` refers to the current scope.
        * @returns {function()} Returns a de-registration function for all listeners.
        */