clarify image change restart behavior for sidecar containers

Author: dshebib

content/en/docs/concepts/workloads/pods/sidecar-containers.md

@@ -121,6 +121,9 @@ Init containers stop before the main containers start up, so init containers can
 exchange messages with the app container in a Pod. Any data passing is one-way
 (for example, an init container can put information inside an `emptyDir` volume).
 
+Changing the image of a sidecar container will not cause the Pod to restart, but will
+trigger a container restart.
+
 ## Resource sharing within containers
 
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