add a comment on how to put a node in maintenance mode

cluster/prod/app/garage/deploy/garage.hcl

@@ -23,6 +23,13 @@ job "garage" {
     }
 
     task "server" {
+      # V---- useful to operate a maintenance on one garage node
+      #       do not forget to check that garage is fully healthy before
+      # constraint {
+      #   attribute = "${attr.unique.hostname}"
+      #   operator = "!="
+      #   value = "pamplemousse"
+      # }
       driver = "docker"
       config {
         image = "dxflrs/garage:v1.99.1-internal"