rustdoc: add an item-info for documented impls

[jsha]

Right now, trait impls on a struct will show one of:

documentation for that specific impl, if available, or
documentation from the trait.
If it's documentation from the trait, we show a short blurb followed by "Read more."

However, this is subtle and implicit. It's not always clear when looking at impl documentation whether it comes from the trait or the impl.

We should favor explicit over implicit: when documentation comes from the impl (the less common case), we should explicitly mark it with an item-info box, saying something like "documentation of this implementation." That can draw attention to impl-specific documentation, which might indicate something unusual about the implementation that could otherwise go unnoticed.