Added note about Docker credentials store (#3456)

* Added note about Docker credentials store

Docker might use a credentials store when logging in to docker repositories, see https://docs.docker.com/engine/reference/commandline/login/#credentials-store. In that case you don't find the `auth` token in the `config.json` file. I added a note that this might happen, in order to prevent confusion.

* added "NOTE:" to the Docker credentials store note

as suggested in PR review

docs/tasks/configure-pod-container/pull-image-private-registry.md

@@ -42,6 +42,8 @@ The output contains a section similar to this:
             }
         }
     }
+    
+NOTE: If you use a Docker credentials store, you won't see that `auth` entry but a `credsStore` entry with the name of the store as value.
 
 ## Creating a Secret that holds your authorization token