<input aria-multiline=true> shouldn't be allowed

[benbeaudry]

This is in direct contradiction with the input element, which by definition is always single line. Nothing specifies in the specs that an author can't set aria-multiline=true on an input element at the moment. Should we add a condition?

[scottaohara]

Transferring this to the ARIA in HTML specification, as that's where this rule (and agreed, it would make sense to add) would need to be defined.