The `Path::join` method is not very discoverable

[albertlarsan68]

Location

std::path::PathBuf::push()'s docs

Summary

There should be a link to the std::path::Path::join() method. It is a method that is already linked in the other way, and can reduce the likelihood of things like

-    let stamp_file = {
-        let mut s = build.out.clone();
-        s.push("rustfmt.stamp");
-        s
-    };
+    let stamp_file = build.out.join("rustfmt.stamp");

happening.

[Ezrashaw]

@rustbot claim