`multiple_supertrait_upcastable` lint example provides `unknown_lints` warning

[laralove143]

Location

https://doc.rust-lang.org/rustc/lints/listing/allowed-by-default.html#example-15

Summary

multiple_supertrait_upcastable lint example is unexpectedly producing a unknown_lints warning

either #![feature(multiple_supertrait_upcastable)] should be added and the documentation should be ran in nightly if not already, or the lint should be made stable