Add notes to stderr of non-exhaustive-reference test

Author: Daniel Noom

src/test/ui/pattern/usefulness/issue-78123-non-exhaustive-reference.rs

@@ -1,7 +1,11 @@
 enum A {}
+    //~^ NOTE `A` defined here
 
 fn f(a: &A) {
-    match a {} //~ ERROR non-exhaustive patterns: type `&A` is non-empty
+    match a {}
+    //~^ ERROR non-exhaustive patterns: type `&A` is non-empty
+    //~| NOTE the matched value is of type `&A`
+    //~| NOTE references are always considered inhabited
 }
 
 fn main() {}

src/test/ui/pattern/usefulness/issue-78123-non-exhaustive-reference.stderr

@@ -1,5 +1,5 @@
 error[E0004]: non-exhaustive patterns: type `&A` is non-empty
-  --> $DIR/issue-78123-non-exhaustive-reference.rs:4:11
+  --> $DIR/issue-78123-non-exhaustive-reference.rs:5:11
    |
 LL | enum A {}
    | --------- `A` defined here