add a note to the error message

src/test/ui/bare-function-self.rs

@@ -1,4 +1,5 @@
 fn a(&self) { }
 //~^ ERROR unexpected `self` argument in bare function
+//~| NOTE invalid argument in bare function
 
 fn main() { }

src/test/ui/bare-function-self.stderr

@@ -1,8 +1,8 @@
 error: unexpected `self` argument in bare function
-  --> $DIR/bare-function-self.rs:12:11
+  --> $DIR/bare-function-self.rs:1:7
    |
 LL | fn a(&self) { }
-   |           ^
+   |       ^^^^ invalid argument in bare function
 
 error: aborting due to previous error