Include documentation of explicit shim trait impls in rustdoc of ExternType

[dtolnay]

The content in https://cxx.rs/extern-c++.html#explicit-shim-trait-impls would be good to copy or at least link from https://docs.rs/cxx/1.0.32/cxx/trait.ExternType.html. The whole point of ExternType is to enable using Rust types defined outside of the current bridge, and requiring some CXX shim trait impl on those types is absolutely the kind of thing that comes up, which makes ExternType documentation a natural place people would think to look for documentation of how to make it work.

Privately shared anecdote:

As a quick suggestion: it might be worth referencing this from the ExternType page that mentions how you can share types across crates (https://docs.rs/cxx/1.0.32/cxx/trait.ExternType.html). I spent a little while trying to figure this out and looking into cxx's code before I eventually found the "declare a useless struct" workaround

The user had discovered that the following results in the right impls they need being generated:

#[cxx::bridge]
mod ffi {
    /* ... */

    struct _ForceCxxImpl {
        f1: UniquePtr<ProcessorEventHandler>,
        f2: SharedPtr<ProcessorEventHandler>,
    }
}

without finding out about the recommended approach of doing:

    impl UniquePtr<ProcessorEventHandler> {}
    impl SharedPtr<ProcessorEventHandler> {}