Remove the note about the difference between open and closed shadow root serialization

This note doesn't really clarify the behavior and just makes things more confusing, we should rather write a proper description.

Author: lutien

index.bs

@@ -2115,10 +2115,6 @@ an |ownership type|, a |serialization internal map|, a |realm| and a |session|:
                   false, |ownership type|, |serialization internal map|,
                   |realm|, and |session|.
 
-                Note: this means the <code>handle</code> for the shadow root
-                will be serialized irrespective of whether the shadow is open or closed,
-                but no properties of the node will be returned.
-
              1. Set |serialized|["<code>shadowRoot</code>"] to |serialized shadow|.
 
       1. If |serialized| is not null, set field <code>value</code> of |remote value| to