[dom] Clarify intro text on non-normative sections

RByers has just created a new issue for https://github.com/whatwg/dom:

== Clarify intro text on non-normative sections ==
>From 
[here](https://github.com/RByers/EventListenerOptions/issues/20#issuecomment-143238689):

@rbyers:
> Ok, thanks. I'm a little confused by [this 
sentence](https://dom.spec.whatwg.org/#conformance):

>> All diagrams, examples, and notes in this specification are 
non-normative, as are all sections explicitly marked non-normative. 
Everything else in this specification is normative.

> Should it be changed (since there aren't actually any sections 
explicitly marked non-normative as far as I can see?).

@annevk:
> We should maybe change that paragraph, yes. Or consistently markup 
non-normative sections I guess. Meh. Follow up issue?

See https://github.com/whatwg/dom/issues/83