Fix several return values documentation

src/core/PaperScript.js

@@ -442,7 +442,7 @@ Base.exports.PaperScript = function() {
      * @param {String} code the PaperScript code
      * @param {PaperScope} scope the scope for which the code is executed
      * @param {Object} [option] the compilation options
-     * @return the exports defined in the executed code
+     * @return {Object} the exports defined in the executed code
      */
     function execute(code, scope, options) {
         // Set currently active scope.

src/item/Item.js

@@ -2832,7 +2832,7 @@ new function() { // Injection scope for hit-test functions shared with project
      * no children, a {@link TextItem} with no text content and a {@link Path}
      * with no segments all are considered empty.
      *
-     * @return Boolean
+     * @return {Boolean}
      */
     isEmpty: function() {
         var children = this._children;

src/item/Project.js

@@ -133,7 +133,7 @@ var Project = PaperScopeItem.extend(/** @lends Project# */{
     /**
      * Checks whether the project has any content or not.
      *
-     * @return Boolean
+     * @return {Boolean}
      */
     isEmpty: function() {
         return !this._children.length;