Rustdoc: distinguish provided methods on trait impls

[alercah]

Currently, if a trait impl uses a provided method's default implementation, there is no easy way to tell directly from the rustdoc. For instance, see the Write for Vec<u8> impl block. The first three methods are implemented by Vec<u8>, while the other two use the default implementation. Of the ones implemented by Vec<u8>, write_all overrides a default implementation while the other two are required.

Ideally, we would have some small way to distinguish between provided, overridden, and required methods in the trait impl blocks. While it isn't useful for normal documentation practices, it is useful for educational purposes (e.g. if I want to see how a type implements a trait, I can know to ignore provided methods) as well as debugging, if I have different levels of trust for the trait author and implementor.