Lint against marking things that wouldn't show up in the documentation anyway doc(hidden).

Examples are:

• Private items.

  #[doc(hidden)]
  fn foo() { }

• Implementing private traits (example lifted from rust-sfml)

        // Wrappable is a private trait that doesn't show up in documentation,
        // so neither will implementations.
    #[doc(hidden)]
    impl Wrappable<*mut ffi::sfConvexShape> for ConvexShape {
        #[doc(hidden)]
        fn wrap(convex_shape: *mut ffi::sfConvexShape) -> ConvexShape {
            ConvexShape {
                convex_shape: convex_shape.clone(),
                texture: None
            }
        }
  
        #[doc(hidden)]
        fn unwrap(&self) -> *mut ffi::sfConvexShape {
            self.convex_shape
        }
    }