Clarify that bs-modal-* classes are only used to target example modals

Created by: gxclarke

The syntax for using modal size options seems inconsistent with the practices used by bootstrap everywhere else:

1. The outer class is "bs-modal-sm|lg". Nowhere else is a "bs-" prefix used for class names.
2. The inner class of "modal-sm|lg" shouldn't be required. CSS rules can handle this easily without the second class.