Add rules to clarify valid aria-invalid use

[scottaohara]

aria-invalid can have the following values:

• true - to indicate the value entered by the user is invalid
• false - default state, no issues with validity
• grammar - a grammatical error was found
• spelling - spelling error found

ARIA 1.2 is deprecating the attribute from being global as the true, and in some ways false, values are specifically suited for form controls. grammar and spelling however, are still necessary for rich text editing, and though the attribute is being deprecated globally, conformance checkers should not outright flag its use on non-form controls to be incorrect.

So, in regards to this spec, we would need to call out that aria-invalid=true|false should be allowed on the form controls where this makes sense, but otherwise (specifically aria-invalid=true) not be allowed on other elements. But aria-invalid=grammar|spelling would still be allowed on any HTML element that otherwise allows for global aria-* attributes.

related to https://github.com/w3c/aria/issues/989

[scottaohara]

Requires issue 391 (linked above) be merged to create the section of the spec to include this information.