fixed .clearfix example, + doc link

The old example is obviously broken, since there seemed to be no need for a `.clearfix`, *until* I read the [official docs](http://getbootstrap.com/css/#grid-responsive-resets).

I took the liberty to link said docs inside the example.  I assume the examples are there to learn from, & deep linking to docs will improve learning.  I hope you'll let this one though, & perhaps encourage more of these links.