test(wgsl): note link b/w cross and bad_cross_builtin_args tests

naga/tests/in/cross.wgsl

@@ -1,3 +1,4 @@
+// NOTE: invalid combinations are tested in the `validation::bad_cross_builtin_args` test.
 @compute @workgroup_size(1) fn main() {
     let a = cross(vec3(0., 1., 2.), vec3(0., 1., 2.));
 }

naga/tests/validation.rs

@@ -264,6 +264,7 @@ fn emit_workgroup_uniform_load_result() {
 #[cfg(feature = "wgsl-in")]
 #[test]
 fn bad_cross_builtin_args() {
+    // NOTE: Things we expect to actually compile are in the `cross` snapshot test.
     let cases = [
         (
             "vec2(0., 1.)",