Documentation text on trait impl methods is hidden in docs

[bluss]

I use documentation on my trait impls to tell users about how my types behave:

impl<N, E, Ty, Ix> Index<NodeIndex<Ix>> for Graph<N, E, Ty, Ix> where
    Ty: EdgeType,
    Ix: IndexType,
{
    type Output = N;
    /// Index the **Graph** by **NodeIndex** to access node weights.
    ///
    /// **Panics** if the node doesn't exist.
    fn index(&self, index: NodeIndex<Ix>) -> &N {
        &self.nodes[index.index()].weight
    }
}

This documentation is not rendered anymore. I expect to see it under the entry for the Index implementation of the Graph type.

I think this was lost after (the otherwise very awesome!) documentation changes by @alexcrichton

[bluss]

Note: Documentation on top of the impl item itself is properly rendered. If you too agree this is adequate, please close the issue again.

[milibopp]

Just for reference: this also leaves the (perhaps not entirely obvious) operator impls on HashSets undocumented. Also see this SO question on this issue.

[mcast]

Yes, #27058 is dup of this. It is hiding stable parts of std from me: https://doc.rust-lang.org/1.1.0/src/core/result.rs.html#883-890