Mention the syntax for use on mod foo; if foo doesn't exist

Newcomers might get confused that `mod` is the only way of defining
scopes, and that it can be used as if it were `use`.

Fix #69492.

compiler/rustc_expand/src/errors.rs

@@ -350,6 +350,7 @@ pub(crate) struct ModuleInBlockName {
 #[derive(Diagnostic)]
 #[diag(expand_module_file_not_found, code = "E0583")]
 #[help]
+#[note]
 pub(crate) struct ModuleFileNotFound {
     #[primary_span]
     pub span: Span,