Does not insert docs into impls on type aliases

[sdroege]

https://github.com/sdroege/gstreamer-rs/tree/dae1a517646ce538383778235ff53e9e973cda4d

docs/gstreamer/docs.md contains docs for e.g. Event::new_eos. This exists in gstreamer/src/event.rs, in the impl GstRc<EventRef>. Event is a type alias to GstRc<EventRef>, and due to a rustdoc bug I can't use that for the impl block (rust-lang/rust#44182).

You can reproduce this by doing cargo build --features embed-lgpl-docs and then seeing how it does not insert the docs from docs.md into the source code.