Add more advice on when not to use Promises

[plinss]

w3ctag/design-principles#138 was caused by a use of Promises that were never expected to resolve. That API has since been redesigned. We should advise against using promises that only reject.

[atanassov]

@plinss and I discussed during our May 2020 virtual f2f. Reading through the original issue and current state of the promises spec we propose adding a new sub-section and recommend that promises are not be used if there is no clear path for completion under normal circumstances.

[annevk]

What's an example of something that almost always throws?

[plinss]

There was a (since changed) API design, IIRC for wake lock, where a promise was used that never resolved but only rejected. We're advising not to do that.