[Edit] Add clarifying comments

Add clarifying comments related to checking of conditions
on unload, WTD-1035.

platform/commonUI/edit/src/controllers/EditController.js

@@ -31,9 +31,19 @@ define(
             });
 
             return {
+                /**
+                 * Get the domain object which is navigated-to.
+                 * @returns {DomainObject} the domain object that is navigated-to
+                 */
                 navigatedObject: function () {
                     return navigatedObject;
                 },
+                /**
+                 * Get the warning to show if the user attempts to navigate
+                 * away from Edit mode while unsaved changes are present.
+                 * @returns {string} the warning to show, or undefined if
+                 *          there are no unsaved changes
+                 */
                 getUnloadWarning: function () {
                     var editorCapability = navigatedObject &&
                             navigatedObject.getCapability("editor"),

platform/commonUI/edit/src/directives/MCTBeforeUnload.js

@@ -43,12 +43,14 @@ define(
                     }
                 }
 
-                // Include this instance of the directive's unload function
-                unloads.push(unload);
-
                 // If this is the first active instance of this directive,
                 // register as the window's beforeunload handler
+                if (unloads.length === 0) {
                     $window.onbeforeunload = checkUnloads;
+                }
+
+                // Include this instance of the directive's unload function
+                unloads.push(unload);
 
                 // Remove it when the scope is destroyed
                 scope.$on("$destroy", removeUnload);