[whatwg] Use IRI instead of URI

Anne van Kesteren wrote:
> 2.5. Extensions to existing attributes
> 
> Under 'maxlength': "Valid URIs and e-mail addresses in particular can
> often be surprisingly long."
> 
> I guess that should be IRI since it refers to the value |type="uri"| can
> have.

    Should that be |type="iri"|??? Perhaps we should have both, with 
"uri" being an alias.