`std::error::Error` docs don't mention `e.downcast()` (only `downcast_{ref,mut}`)

[thomcc]

I didn't notice an issue on file for this already, feel free to dupe against it if one exists.

In #99917 we landed support for error in core, and sadly one of the casualties was the docs got somewhat worse. Concretely, Box<dyn Error + stuff>::downcast() is no longer shown on the docs page for Error, which is a bit of a bummer, since these methods do exist: https://doc.rust-lang.org/nightly/src/alloc/boxed.rs.html#2127

[jyn514]

I don't think rustdoc supports incoherent impls like this except for primitives.

[thomcc]

That's unfortunate. Error is an important trait, and its docs are notably worse after that PR. If supporting that is too hard, it might be worth special-casing it (Failing that, manually documenting this stuff is an option I suppose).