Handle cross-provider middleware in kubernetes CRD

Co-authored-by: Julien Salleyron <julien.salleyron@gmail.com>

docs/content/middlewares/overview.md

@@ -79,32 +79,6 @@ labels:
   - "traefik.http.router.router1.Middlewares=foo-add-prefix@rancher"
 ```
 
-```yaml tab="Kubernetes"
-apiVersion: apiextensions.k8s.io/v1beta1
-kind: CustomResourceDefinition
-metadata:
-  name: tlsoptions.traefik.containo.us
-
-spec:
-  group: traefik.containo.us
-  version: v1alpha1
-  names:
-    kind: TLSOption
-    plural: tlsoptions
-    singular: tlsoption
-  scope: Namespaced
-
----
-apiVersion: traefik.containo.us/v1alpha1
-kind: TLSOption
-metadata:
-  name: mytlsoption
-  namespace: default
-
-spec:
-  minversion: VersionTLS12
-```
-
 ```toml tab="File"
 # As Toml Configuration File
 [providers]
@@ -133,35 +107,66 @@ spec:
 When you declare a middleware, it lives in its provider namespace.
 For example, if you declare a middleware using a Docker label, under the hoods, it will reside in the docker provider namespace.
 
-If you use multiple providers and wish to reference a middleware declared in another provider,
-then you'll have to prefix the middleware name with the provider name.
+If you use multiple providers and wish to reference a middleware declared in another provider
+(aka referencing a cross-provider middleware),
+then you'll have to append to the middleware name, the `@` separator, followed by the provider name.
 
 ```text
 <resource-name>@<provider-name>
 ```
 
+!!! important "Kubernetes Namespace"
+
+	As Kubernetes also has its own notion of namespace, one should not confuse the "provider namespace"
+with the "kubernetes namespace" of a resource when in the context of a cross-provider usage.
+In this case, since the definition of the middleware is not in kubernetes,
+specifying a "kubernetes namespace" when referring to the resource does not make any sense,
+and therefore this specification would be ignored even if present.
+
 !!! abstract "Referencing a Middleware from Another Provider"
 
     Declaring the add-foo-prefix in the file provider.
 
     ```toml
     [providers]
-       [providers.file]
+      [providers.file]
 
     [http.middlewares]
-     [http.middlewares.add-foo-prefix.AddPrefix]
+      [http.middlewares.add-foo-prefix.AddPrefix]
         prefix = "/foo"
     ```
 
-    Using the add-foo-prefix middleware from docker.
+    Using the add-foo-prefix middleware from other providers:
 
-    ```yaml
+    ```yaml tab="Docker"
     your-container: #
-        image: your-docker-image
+      image: your-docker-image
 
-        labels:
-          # Attach add-foo-prefix@file middleware (declared in file)
-          - "traefik.http.routers.my-container.middlewares=add-foo-prefix@file"
+      labels:
+        # Attach add-foo-prefix@file middleware (declared in file)
+        - "traefik.http.routers.my-container.middlewares=add-foo-prefix@file"
+    ```
+
+    ```yaml tab="Kubernetes"
+    apiVersion: traefik.containo.us/v1alpha1
+    kind: IngressRoute
+    metadata:
+      name: ingressroutestripprefix
+
+    spec:
+      entryPoints:
+        - web
+      routes:
+        - match: Host(`bar.com`)
+          kind: Rule
+        services:
+          - name: whoami
+            port: 80
+        middlewares:
+          - name: add-foo-prefix@file
+          # namespace: bar
+          # A namespace specification such as above is ignored
+          # when the cross-provider syntax is used.
     ```
 
 ## Available Middlewares