Add a better help message for kubectl describe

When resource type isn't given in the command, print the possible
resource types. Also added the resource type description in help
strings.

pkg/kubectl/cmd/cmd.go

@@ -90,6 +90,18 @@ __custom_func() {
             ;;
     esac
 }
+`
+	valid_resources = `Valid resource types include:
+   * pods (aka 'po')
+   * replicationcontrollers (aka 'rc')
+   * services (aka 'svc')
+   * nodes (aka 'no')
+   * events (aka 'ev')
+   * secrets
+   * limits
+   * persistentVolumes (aka 'pv')
+   * persistentVolumeClaims (aka 'pvc')
+   * quota
 `
 )