[whatwg/webidl] Throwing DOMExceptions in the context of a promise returning operation (Issue #1301)

(little bit of a rehash of https://github.com/whatwg/webidl/issues/214)

As part of feedback for this Screen Orientation spec pull request: 
https://github.com/w3c/screen-orientation/pull/251/files#r1180457386

There is a promise-returning operation (`.lock()`). In that context,@annevk correctly pointed out that I could simply write something like:

 * If condition, [=exception/throw=] a newly created "WhateverError" {{DOMException}}. 

And have the IDL binding layer do the right thing, as it knows that the [operation returns a promise](https://heycam.github.io/webidl/#dfn-create-operation-function). 

However, the definition of [throw](https://webidl.spec.whatwg.org/#dfn-throw) doesn't make it obviously clear to the reader, who may not know WebIDL very well, that the binding layer will automatically handle the "throw" as a promise rejection. 

I wonder if it might be helpful to write a note to describe this behavior and some examples? I've personally experienced issues with spec reviewers being confused if an operation is actually going to throw an exception and not return a rejected promise (just due to this behavior being a bit "magical").  

At the same time, it is nice that we have the very clear "return a promise rejected with" to fall back on. 


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