rustdoc should copy docs from interfaces to implementation methods

[brson]

Undocumented methods should have their docs copied from the interface documentation. This will be difficult for interfaces in the same crate, and very difficult for interfaces in external crates.

[graydon]

Nominating for feature completeness.

[brson]

Could also not specify the methods on trait impls, and just link back to the trait.

[graydon]

accepted for feature-complete milestone

[msullivan]

No change, I think. Part of #5413

[emberian]

#8125

Fixed by rustdoc_ng

/cc @Seldaek

[emberian]

Now that rustdoc_ng has been merged, closing.