Extend let-unit-in semgrep rule message to warn about parentheses

Co-authored-by: Michael Schwarz <[email protected]>

Author: sim642

.semgrep/let.yml

@@ -1,6 +1,6 @@
 rules:
   - id: let-unit-in
     pattern: let () = $E in ...
-    message: use ; instead
+    message: use ; instead (and, if needed, add surrounding parentheses to preserve precedence)
     languages: [ocaml]
     severity: WARNING