Validate image name for kubectl rolling-update NAME --image=IMAGE

Require distinct image from current one when starting a new
rolling-update, and exit with error if an existing in-progress update
is targeting a different image.

docs/user-guide/kubectl/kubectl_rolling-update.md

@@ -72,7 +72,7 @@ $ kubectl rolling-update frontend --image=image:v2
       --deployment-label-key="deployment": The key to use to differentiate between two different controllers, default 'deployment'.  Only relevant when --image is specified, ignored otherwise
       --dry-run[=false]: If true, print out the changes that would be made, but don't actually make them.
   -f, --filename=[]: Filename or URL to file to use to create the new replication controller.
-      --image="": Image to use for upgrading the replication controller.  Can not be used with --filename/-f
+      --image="": Image to use for upgrading the replication controller. Must be distinct from the existing image (either new image or new image tag).  Can not be used with --filename/-f
       --no-headers[=false]: When using the default output, don't print headers.
   -o, --output="": Output format. One of: json|yaml|wide|name|go-template=...|go-template-file=...|jsonpath=...|jsonpath-file=... See golang template [http://golang.org/pkg/text/template/#pkg-overview] and jsonpath template [http://releases.k8s.io/HEAD/docs/user-guide/jsonpath.md].
       --output-version="": Output the formatted object with the given version (default api-version).
@@ -119,7 +119,7 @@ $ kubectl rolling-update frontend --image=image:v2
 
 * [kubectl](kubectl.md)	 - kubectl controls the Kubernetes cluster manager
 
-###### Auto generated by spf13/cobra on 9-Oct-2015
+###### Auto generated by spf13/cobra on 19-Oct-2015
 
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