Clarify allowed attributes for input type=number

[scottaohara]

Per w3c/aria#1716, an input type=number has an implicit role of spinbutton. Indicating an input type=number can use all the allowed aria-* attributes for a spinbutton will likely create inconsistencies / browser corrections for ARIA attributes conflicting with the implicit properties of the input field.

todo: perform tests and indicate which attributes, if any, should not be allowed for authors to specify.

[scottaohara]

This issue likely extends beyond just this particular role/input