[BackToOpener] Make a stronger case for this being opt-in

[domenic]

I think the biggest question mark about this proposal is whether it should be opt-in or applied automatically across the entire user agent.

The explainer already discusses this somewhat. However, I don't find its logic that convincing. Many of the arguments, especially in https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/BackToOpener/explainer.md#user-facing-problem, for why this is important to solve, steer me in the direction of it applying everywhere.

Note that some user agents, e.g. Chrome for Android, already apply this behavior across the entire user agent.

I would be most convinced if you had clear examples of cases where this behavior would be confusing or harmful.