show/hide “allowed html” text

i’ll swear that somewhere i saw a wordpress site that had a link to show / hide the “allowed html” text just before the comments box. i’d love to have that functionality on my site, but can’t find an example of it anywhere. did i imagine it? does anyone have a link, or better still, know of a plugin. i’ve searched but no joy, it’s late though, so sorry if i missed something obvious.

matthew