[Forms] Add clarifying comments

Add in-line documentation to directives and
controller used for the Forms component. WTD-530.

platform/forms/src/controllers/DateTimeController.js

@@ -6,8 +6,17 @@ define(
 
         var DATE_FORMAT = "YYYY-DDD";
 
+        /**
+         * Controller for the `datetime` form control.
+         * This is a composite control; it includes multiple
+         * input fields but outputs a single timestamp (in
+         * milliseconds since start of 1970) to the ngModel.
+         *
+         * @constructor
+         */
         function DateTimeController($scope) {
 
+            // Update the
             function update() {
                 var date = $scope.datetime.date,
                     hour = $scope.datetime.hour,
@@ -23,6 +32,7 @@ define(
                 }
             }
 
+            // Update value whenever any field changes.
             $scope.$watch("datetime.date", update);
             $scope.$watch("datetime.hour", update);
             $scope.$watch("datetime.min", update);