Add kubectl diff exit code doc

staging/src/k8s.io/kubectl/docs/book/pages/app_composition_and_deployment/diffing_local_and_remote_resources.md

@@ -42,3 +42,17 @@ export KUBECTL_EXTERNAL_DIFF=meld; kubectl diff -k ./dir/
 
 {% endmethod %}
 
+{% method %}
+## Exit status
+The following exit values shall be returned:
+
+ `0`
+No differences were found.
+ `1`
+Differences were found.
+ `>1`
+Kubectl or diff failed with an error.
+
+**Note:** `KUBECTL_EXTERNAL_DIFF`, if used, is expected to follow that convention.
+
+{% endmethod %}

staging/src/k8s.io/kubectl/pkg/cmd/diff/diff.go

@@ -56,7 +56,18 @@ var (
 
 		KUBECTL_EXTERNAL_DIFF environment variable can be used to select your own
 		diff command. By default, the "diff" command available in your path will be
-		run with "-u" (unified diff) and "-N" (treat absent files as empty) options.`))
+		run with "-u" (unified diff) and "-N" (treat absent files as empty) options.
+		
+		Exit status:
+		 0
+		No differences were found.
+		 1
+		Differences were found.
+		 >1
+		Kubectl or diff failed with an error.
+
+		Note: KUBECTL_EXTERNAL_DIFF, if used, is expected to follow that convention.`))
+
 	diffExample = templates.Examples(i18n.T(`
 		# Diff resources included in pod.json.
 		kubectl diff -f pod.json