Address blackhat webperf ideas

Thanks to Zach for kicking off a worthwhile thread: https://twitter.com/zachleat/status/1122184546609446919 😉

* cloaking: we need to publish docs on this
* [``](https://twitter.com/mmatuzo/status/1122375038236332043)
* [sync dialogs don't affect perf](https://twitter.com/zachleat/status/1105912986504437760).

Review the linked Zach Leatherman and Manuel Matuzović discussions, then determine which of the cloaking, aria-hidden body, and synchronous-dialog claims Lighthouse should address. Done requires an agreed scope and a clear decision about whether each item needs documentation, an audit, or no change.

Written by the indexing model from the issue text.