Add note to avoid confusing

Author: JohnTitor

doc/adding_lints.md

@@ -86,6 +86,8 @@ test. That allows us to check if the output is turning into what we want.
 
 Once we are satisfied with the output, we need to run
 `tests/ui/update-all-references.sh` to update the `.stderr` file for our lint.
+Please note that, we should run `TESTNAME=ui/foo_functions cargo uitest`
+every time before running `tests/ui/update-all-references.sh`.
 Running `TESTNAME=ui/foo_functions cargo uitest` should pass then. When we
 commit our lint, we need to commit the generated `.stderr` files, too.