[dom] Clarify intro text on non-normative sections (#83)

>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?

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