kubeadm-upgrade: include generated page about 'diff'

Also change the note about the how-to-upgrade guides.

content/en/docs/reference/setup-tools/kubeadm/kubeadm-upgrade.md

@@ -17,7 +17,7 @@ cluster if necessary.
 ## kubeadm upgrade guidance
 
 Every upgrade process might be a bit different, so we've documented each minor upgrade process individually.
-Please check these documents out for more detailed how-to-upgrade guidance:
+For more version-specific upgrade guidance, see the following resources:
 
  * [1.6 to 1.7 upgrades](/docs/tasks/administer-cluster/kubeadm-upgrade-1-7/)
  * [1.7.x to 1.7.y upgrades](/docs/tasks/administer-cluster/kubeadm-upgrade-1-8/)
@@ -33,6 +33,9 @@ Please check these documents out for more detailed how-to-upgrade guidance:
 ## kubeadm upgrade apply  {#cmd-upgrade-apply}
 {{< include "generated/kubeadm_upgrade_apply.md" >}}
 
+## kubeadm upgrade diff {#cmd-upgrade-diff}
+{{< include "generated/kubeadm_upgrade_diff.md" >}}
+
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