Add snake case lint note about keyword identifiers which cannot be raw

compiler/rustc_lint/src/nonstandard_style.rs

@@ -283,6 +283,7 @@ impl NonSnakeCase {
                             if sc_ident.name.can_be_raw() {
                                 ("rename the identifier or convert it to a snake case raw identifier", sc_ident.to_string())
                             } else {
+                                err.note(&format!("`{}` cannot be used as a raw identifier", sc));
                                 ("rename the identifier", String::new())
                             }
                         } else {

src/test/ui/lint/lint-non-snake-case-identifiers-suggestion-reserved.stderr

@@ -60,6 +60,8 @@ error: variable `Super` should have a snake case name
    |
 LL |     let Super: usize = 0;
    |         ^^^^^ help: rename the identifier
+   |
+   = note: `super` cannot be used as a raw identifier
 
 error: aborting due to 4 previous errors; 2 warnings emitted