docs($http): add a note about modifying data in transformRequest

Closes #12468

src/ng/http.js

@@ -550,6 +550,15 @@ function $HttpProvider() {
      * the transformed value (`function(data, headersGetter, status)`) or an array of such transformation functions,
      * which allows you to `push` or `unshift` a new transformation function into the transformation chain.
      *
+     * <div class="alert alert-warning">
+     * **Note:** Angular does not make a copy of the `data` parameter before it is passed into the `transformRequest` pipeline.
+     * That means changes to the properties of `data` are not local to the transform function (since Javascript passes objects by reference).
+     * For example, when calling `$http.get(url, $scope.myObject)`, modifications to the object's properties in a transformRequest
+     * function will be reflected on the scope and in any templates where the object is data-bound.
+     * To prevent his, transform functions should have no side-effects.
+     * If you need to modify properties, it is recommended to make a copy of the data, or create new object to return.
+     * </div>
+     *
      * ### Default Transformations
      *
      * The `$httpProvider` provider and `$http` service expose `defaults.transformRequest` and