Add description to the user block selection box

This patch adds a quick description of what blocks are to the block selection box in a user's profile (block_user() function).

Enable or disable extra boxes of information to display beside the main content of the site.

I'm not entirely sure how best to describe blocks without using examples (that may not be there) on that specific site. It used to be that they were boxes that went either side of the content, in one of the sidebars, but they could now be anywhere on the site so that distinction isn't so clear any more.

(patch is against 4.7)