change extra small devices to >=480px in css.html doc

Hey,

I'm refering on this table: [http://getbootstrap.com/css/#responsive-utilities-classes](http://getbootstrap.com/css/#responsive-utilities-classes)

I think that is currently bad documentation since there is by default a media query breakpoint at 480px, which is not represented.

Maybe there should even be a 5th column in that table for all columns below 480px without a class.

What do you think?