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.

Fixes #15109.