doc: update definition for namespaces

Update namespace definition to not use _Virtual Clusters_ as a means of explaining namespaces.

Motif: KubeCon NA 21 Talk on ***"Beyond Namespaces: Virtual Clusters are the Future of Multi-Tenancy - Lukas Gentele, Loft Labs"***

Author: Shabirmean

content/en/docs/concepts/overview/working-with-objects/namespaces.md

@@ -10,8 +10,7 @@ weight: 30
 
 <!-- overview -->
 
-Kubernetes supports multiple virtual clusters backed by the same physical cluster.
-These virtual clusters are called namespaces.
+Namespaces in Kubernetes provides a mechanism for scoping cluster resources by name. Names of resources need to be unique within a namespace, but not across namespaces. Namespace based scoping is applicable only for namespaced objects _(e.g. Deployments, Services, etc)_ and not for cluster-wide objects _(e.g. StorageClass, Nodes, PersistentVolumes, etc)_.
 
 <!-- body -->