feat(functions): add missing region tag for documentation (#13610)

* feat(functions): add missing region tag for documentation

* fix: add a missing nl

Author: iennae

functions/billing_stop_on_notification/requirements.txt

@@ -1,3 +1,5 @@
+# [START functions_billing_stop_requirements]
 functions-framework==3.*
 google-cloud-billing==1.16.2
 google-cloud-logging==3.12.1
+# [END functions_billing_stop_requirements]