[XBL Primer] Comment on structure

The Section 2.2. in the primer is a good example what kind information
an author needs when she familiarize herself with a new language. Often
things like that are scattered around the spec. IMO, there could be more 
sections like that in the primer.

For instance, the author may want to know what are the options to use
the content (the child elements, the textual content, and the
attributes) of the bound element in a shadow tree. This is explained in
the spec but not in a single chapter.

Mikko