Identify non-content attributes which win out vs corresponding aria- attributes

[scottaohara]

for instance, showModal for dialogs should win out vs aria-modal=false
the intermittent idl attribute for checkboxes would win out vs aria-checked=mixed

this might require further progress on the AOM, however, before conformance checkers can accurately determine if these are in conflict. but, for starters we could at least make an informative declaration regarding this, referencing the ARIA spec when it is updated to indicate this rule. see w3c/aria#1610