Configure Alt Text Description

I think this was a feature in D6 but didn't seem to get moved over. After adding an imagefield and having it so users can specify their own alt text, you used to be able to control what the description under the alt text field said. Now it's just defaulting to "This text will be used by screen readers, search engines, or when the image cannot be loaded." Aside from wondering if that feature could be added again, I'm wondering how I can manually change that text without hacking core. I've tried a hook_form_alter and it doesn't show that field.