Docs clean up - unnecessary role attributes, callout clarifications

The removal of role="form" may be considered contentious. It seems that, compared to the clear-cut case of <nav> not needing role="navigation", <form> does not explicitly get a role of form (at least, it doesn't seem to be exposed in my quick testing). So, having role="form" *can* be useful, but only in cases where the author wants to make a form an explicit important landmark. For this reason (requires explicit author intent), I'd leave the attribute out from the general examples here.