this commit makes `deref_into_dyn_supertrait` lint allow-by-default,
removes future incompatibility (we finally live in a broken world), and
changes the wording in the documentation.
previously documentation erroneously said that it lints against *usage*
of the deref impl, while it actually (since 104742) lints on the impl
itself (oooops, my oversight, should have updated it 2+ years ago...)
