docs(reconciler): output value description

reconciler/outputs.tf

 output "stable_revisions" {
-  value = { for service in google_cloud_run_v2_service.runway_service : service.location => tolist([for each in service.traffic : each.revision if each.tag == "stable"])[0] }
+  value       = { for service in google_cloud_run_v2_service.runway_service : service.location => tolist([for each in service.traffic : each.revision if each.tag == "stable"])[0] }
+  description = "The mapping of Cloud Run region to current stable Cloud Run revision"
 }