also ignore doctests

Author: RalfJung

src/tools/tidy/src/error_codes.rs

@@ -357,7 +357,8 @@ fn check_error_codes_used(
             errors.push(format!(
                 "Error code `{code}` exists, but is not emitted by the compiler!\n\
                 Please mark the code as no longer emitted by adding the following note to the top of the `EXXXX.md` file:\n\
-                `#### Note: this error code is no longer emitted by the compiler`"
+                `#### Note: this error code is no longer emitted by the compiler`\n\
+                Also, do not forget to mark doctests that no longer apply as `ignore (error is no longer emitted)`."
             ));
         }