clarify when user will see warning on clicking file -> new

src/components/menu-bar/menu-bar.jsx

@@ -159,7 +159,10 @@ class MenuBar extends React.Component {
     handleClickNew () {
         let readyToReplaceProject = true;
         // if the project is dirty, and user owns the project, we will autosave.
-        // but if they don't own this project, user should consider downloading first.
+        // but if they are not logged in and can't save, user should consider
+        // downloading or logging in first.
+        // Note that if user is logged in and editing someone else's project,
+        // they'll lose their work.
         if (this.props.projectChanged && !this.props.canCreateNew) {
             readyToReplaceProject = confirm( // eslint-disable-line no-alert
                 this.props.intl.formatMessage(messages.confirmNav)