Document impl block visibility

[Subsentient]

It seems that if the type is visible, an impl's pub methods can be used, even if such an impl is encased in multiple private modules.

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2015&gist=bf21f358b4b57075d1243a3b07d93da9

I cannot find anywhere in the documentation, hell, even Google, where this behaviour is documented correctly. Only after opening #79876 did I find out that this is the expected behaviour.

[pierwill]

Would this be something for the API docs here, or the Reference, or both? 🤔

[hkBst]

The path to an associated item is any path to the implementing type, followed by the associated item’s identifier as the final path component.Rust reference

I think that would imply that it does not matter where the impl block is, since the path to its items only depends on where the type is defined.

But https://doc.rust-lang.org/std/keyword.impl.html could I think use some clarification.