[CSS21] backgrounds and page margins clarification

We need to clarify that the canvas background covers the
page area but is not painted in the page margin area.

Proposal:

   In Section 13.2 http://www.w3.org/TR/CSS21/page.html#page-box
   Add to the first bullet
     The canvas background is painted within and covers the page area.
   Add to the second bullet
     The page margin area is transparent.

~fantasai