Then I wonder why there was a bug if it was as simple as to read http://platform.xwiki.org/xwiki/bin/view/DevGuide/SpecialCSSClasses ? :)

The goal of the comment was not to explain what clearfloats does (that's documented, as you mentioned) but to explain why we need it here!

Anyway... it already cost us 5 times the cost of having put that comment... ;)