[BackToOpener] Be clearer about developer-observable effects

[domenic]

It isn't clear if this proposal is purely user-facing, or also developer-facing.

https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/BackToOpener/explainer.md#privacy-and-security-considerations states

the opener's URL is added to the destination browsing context's history

which implies that history.length will become 2, and history.back() will navigate back to the opener URL. (In the current browsing context/tab, I guess?)

However, I think it would be better if this feature were solely user-facing. Instead of modifying anything about developer-facing session history, it would solely modify the browser's back button behavior.