rustdoc: intra-doc-links disambiguation warning doesn't listen to -Dwarnings

[Nemo157]

I expected this to error out, but it didn't:

> RUSTDOCFLAGS=-Dwarnings cargo doc --no-deps -p futures-util-preview
 Documenting futures-util-preview v0.3.0-alpha.1 (file:///Users/Nemo157/sources/futures-rs/futures-util)
warning: `empty` is both a type and a function
 --> futures-util/src/future/empty.rs:6:1
  |
6 | / /// A future which is never resolved.
7 | | ///
8 | | /// This future can be created with the [`empty`] function.
  | |___________________________________________________________^
  |
  = help: try `type@empty` if you want to select the type, or `empty()` if you want to select the function

[...]

    Finished dev [unoptimized + debuginfo] target(s) in 3.26s