update-dbus-docs: add hint
diff --git a/tools/update-dbus-docs.py b/tools/update-dbus-docs.py
index 0bbc4aa..3500d9c 100755
--- a/tools/update-dbus-docs.py
+++ b/tools/update-dbus-docs.py
@@ -308,4 +308,5 @@
         print(f'{p:{mlen + 1}} {t - m}/{t} {c}')
 
     if opts.test and outdated:
-        exit(f'Outdated pages: {", ".join(outdated)}')
+        exit(f'Outdated pages: {", ".join(outdated)}\n'
+             f'Hint: ninja -C {opts.build_dir} man/update-dbus-docs')