Not all trait implementations are documented

[jethrogb]

When implementing traits with type parameters (with your types) for types outside your crate, these aren't documented anywhere.

For example:

pub struct A;

impl From<A> for std::io::Error {
    fn from(a: A) -> Self {
        unimplemented!()
    }
}

This isn't documented in my crate, and it of course can't be documented in std, because std doesn't know about my type.

[QuietMisdreavus]

Duplicate of #25264 (at least my understanding of that issue)