doc: comment pod_exec.py for multiple containers

The behavior in kubectl is to run the exec command
against the default container. However, this needs to
be specified with the client call.

Add a comment in the example for this.

examples/pod_exec.py

@@ -71,6 +71,8 @@ def exec_commands(api_instance):
         '/bin/sh',
         '-c',
         'echo This message goes to stderr; echo This message goes to stdout']
+    # When calling a pod with multiple containers running the target container
+    # has to be specified with a keyword argument container=<name>.
     resp = stream(api_instance.connect_get_namespaced_pod_exec,
                   name,
                   'default',