[CSS3 Paged Media] containing block for margin boxes

The containing block for margin boxes is currently undefined.
Here's some proposed text:

  | The containing block for the corner margin boxes is the rectangle
  | defined by the intersection of the two page margins meeting at
  | that corner. For all other margin boxes, the containing block is
  | the rectangle formed by the page margin area on that side minus
  | the containing blocks of the adjacent corners' margin boxes.

~fantasai