XForms 2.0: @iterate

I found a couple of missing spots in the existing @iterate prose in the 
spec, in section
http://www.w3.org/MarkUp/Forms/wiki/XForms_2.0#XForms_Actions

I added @iterate to the content model of XForms Actions, because it was 
missing from that table.

I also fixed broken links to section "Iteration of XForms Actions" and 
made them point to "The While Attribute" and "The Iterate Attribute" as 
appropriate.