review edits: declarative dialog popover control

[dvdherron]

Closes #332

• doesn't address the splitting of the guides
• begins to add more specific ARIA guidance
• the css issue was fixed in a previous PR
• address the language around fallback usage
• does not update graders/evals

[dvdherron]

@jamesnw I'm a little unsure about what specific guidance makes sense for ARIA usage in the invoker-polyfill fallback. There's a partially working implementation in the demo. See #795 for how it's handled for custom actions.

(Assigning you, just to keep an eye on this while I'm out)