This patch adds descriptive text to the Multiple Values checkbox.

Here's a usability patch for adding some descriptive text to the Multiple Values checkbox, since it's one of the most misunderstood items on the content_admin pages. I've added a #description that says:

Allows users to select more than one of this item. Commonly used for Select Lists where users choose several items at random.

I tried for half an hour to condense the Handbook page description into this short sentence. Anybody want to comment on the text before it gets committed?

(There is no functional code in this patch that requires testing or review)