This allows impls such as `impl QueryData for OneOf<(T,)>` to be displayed as variadic: `impl QueryData for OneOf<(T₁, T₂, …, Tₙ)>`. See question on zulip: <https://rust-lang.zulipchat.com/#narrow/channel/266220-t-rustdoc/topic/Make.20.60.23.5Bdoc.28fake_variadic.29.5D.60.20more.20useful>