Fixup docs for the ingress-class flag.

According to this issue, there is no way to handle all ingress classes.

#3101

cmd/nginx/flags.go

@@ -60,7 +60,7 @@ requests to the first port of this Service.`)
 		ingressClass = flags.String("ingress-class", "",
 			`Name of the ingress class this controller satisfies.
 The class of an Ingress object is set using the field IngressClassName in Kubernetes clusters version v1.18.0 or higher or the annotation "kubernetes.io/ingress.class" (deprecated).
-All ingress classes are satisfied if this parameter is not set.`)
+If this parameter is not set it will handle ingresses with either an empty or "nginx" class name.`)
 
 		configMap = flags.String("configmap", "",
 			`Name of the ConfigMap containing custom global configurations for the controller.`)

docs/user-guide/cli-arguments.md

@@ -25,7 +25,7 @@ They are set in the container spec of the `nginx-ingress-controller` Deployment
 | `--healthz-port`                   | Port to use for the healthz endpoint. (default 10254) |
 | `--http-port`                      | Port to use for servicing HTTP traffic. (default 80) |
 | `--https-port`                     | Port to use for servicing HTTPS traffic. (default 443) |
-| `--ingress-class`                  | Name of the ingress class this controller satisfies. The class of an Ingress object is set using the field IngressClassName in Kubernetes clusters version v1.18.0 or higher or the annotation "kubernetes.io/ingress.class" (deprecated). All ingress classes are satisfied if this parameter is not set. |
+| `--ingress-class`                  | Name of the ingress class this controller satisfies. The class of an Ingress object is set using the field IngressClassName in Kubernetes clusters version v1.18.0 or higher or the annotation "kubernetes.io/ingress.class" (deprecated). If this parameter is not set it will handle ingresses with either an empty or "nginx" class name. |
 | `--kubeconfig`                     | Path to a kubeconfig file containing authorization and API server information. |
 | `--log_backtrace_at`               | when logging hits line file:N, emit a stack trace (default :0) |
 | `--log_dir`                        | If non-empty, write log files in this directory |