Point out that behavior might be switched on 2015 and 2018 too one day
Reword documentation to make it clear that behaviour can be switched on older editions too, one day in the future. It doesn't *have* to be switched, but I think it's good to have it as an option and re-evaluate it a few months/years down the line when e.g. the crates that showed up in crater were broken by different changes in the language already.
cc #25725, #65819, #66145, #84147 , and https://github.com/rust-lang/rust/issues/84133#issuecomment-818005314
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