[whatwg] content models clarity

[Rikkert Koppes]

The content models [1] section is pretty clear, however, whenever one 
wants to know which elements fall into a specific content model, one has 
to look at the "categories" definition of each element.

A (possibly non normative) overview would be helpful. I understand it is 
repeating information, hence possibly leading to conflicts when not 
paying attention while editing, but I think it would clarify a lot. 
Consider a similar overview to the one found in [2] listing the 
applicable attributes to various input types.

[1] 
http://www.whatwg.org/specs/web-apps/current-work/multipage/dom.html#content-models
[2] 
http://www.whatwg.org/specs/web-apps/current-work/multipage/forms.html#the-input-element

Regards,
Rikkert Koppes