`multiple_supertrait_upcastable` lint example provides `unknown_lints` warning

### 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