rustdoc: change error message for invalid `#[doc(tuple_variadic)]`

Add test case.

compiler/rustc_passes/src/check_attr.rs

@@ -815,7 +815,7 @@ impl CheckAttrVisitor<'_> {
                         .sess
                         .struct_span_err(
                             meta.span(),
-                            "`#[doc(tuple_variadic)]` can only be used on unary tuples",
+                            "`#[doc(tuple_variadic)]` must used on the first of a set of tuple trait impls with varying arity",
                         )
                         .emit();
                     return false;

src/test/rustdoc-ui/tuple-variadic-check.rs

@@ -0,0 +1,15 @@
+#![feature(rustdoc_internals)]
+
+trait Mine {}
+
+// This one is fine
+#[doc(tuple_variadic)]
+impl<T> Mine for (T,) {}
+
+trait Mine2 {}
+
+// This one is not
+#[doc(tuple_variadic)] //~ ERROR
+impl<T, U> Mine for (T,U) {}
+
+fn main() {}

src/test/rustdoc-ui/tuple-variadic-check.stderr

@@ -0,0 +1,8 @@
+error: `#[doc(tuple_variadic)]` must used on the first of a set of tuple trait impls with varying arity
+  --> $DIR/tuple-variadic-check.rs:12:7
+   |
+LL | #[doc(tuple_variadic)]
+   |       ^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+