definition tweaks

As discussed on the call today, I've made a few minor adjustments to the
definitions:[1]

 

-          there's an editor's note at the start indicating that the
terminology contains some assumptions

-          "static progression" is now "specific progression" in the default
reading order definition

-          "secondary resource" has been split off from primary to avoid
confusion about whether primary resources have secondary

 

As always, feedback on these changes is welcome.

 

[1] https://w3c.github.io/wpub/#terminology

 

Matt