Better Documentation on the new grid sizes classes.

Created by: Bartmax

Since the release of BS3 I seeing more questions about how to apply all column sizes to a div or examples with all column sizes like :

<div class="col-lg-4 col-md-4 col-sm-6 col-xs-6"></div>

I think is a good idea make a point in docs exactly informing users there's no need to do that because BS3 bubble up the column size automagically.