Apply suggestions from code review

Co-authored-by: Urgau <3616612+Urgau@users.noreply.github.com>

Author: CoCo-Japan-pan

compiler/rustc_parse/src/errors.rs

@@ -1269,11 +1269,12 @@ pub(crate) struct IncorrectVisibilityRestriction {
 }
 
 #[derive(Diagnostic)]
-#[diag("incorrect impl restriction")]
+#[diag("incorrect `impl` restriction")]
 #[help(
-    "some possible impl restrictions are:
+    "some possible `impl` restrictions are:
     `impl(crate)`: can only be implemented in the current crate
     `impl(super)`: can only be implemented in the parent module
+    `impl(self)`: can only be implemented in current module
     `impl(in path::to::module)`: can only be implemented in the specified path"
 )]
 pub(crate) struct IncorrectImplRestriction {