Clarifications for HTML's autocomplete and aria-autocomplete

[scottaohara]

Closes #2337

• updates autocomplete mapping for the form element (it doesn't map directly to the form element, but rather owned form controls will expose a supports autocompletion mapping, or not).
• Calls out that aria-autocomplete is not supported on the form element or elements with role=form
• identify that it can be ok to use aria-autocomplete=" [ valid value ] " and autocomplete=off together, but that for autocomplete=" [ valid value ] " and aria-autocomplete="[ valid value ] ", UA behavior should win out.

not sure how much more detail needs to be provided, as attempting to do so felt like treading into repeating what is already defined in each spec for these attributes (html / aria).

Test, Documentation and Implementation tracking

Once this PR has been reviewed and has consensus from the working group, tests should be written and issues should be opened on browsers. Add N/A and check when not applicable.

• [ ] "author MUST" tests:
• [ ] "user agent MUST" tests:
• [ ] Browser implementations (link to issue or commit):
  • WebKit:
  • Gecko:
  • Blink:
• [ ] Does this need AT implementations?
• [ ] Related APG Issue/PR:
• [ ] MDN Issue/PR: