You can. Before clicking your bookmarklet, just highlight the text you want to save on the page. (Put your cursor at the beginning or end of the text, hold down the left button and drag it over the text you want to save. Let go, and the text should remain highlighted.)  When you click the bookmarklet, the higlighted text will automatically populate the Description. (If you do not highlight text, the Description will be pulled from information the site provides.)