Current node is returned if it is a root, not null (dotnet#8663)

Author: arviedelgado

xml/System.Text.Json.Nodes/JsonNode.xml

@@ -2233,8 +2233,7 @@ This method makes a copy of the data the reader acted on, so there is no caller
         <ReturnType>System.Text.Json.Nodes.JsonNode</ReturnType>
       </ReturnValue>
       <Docs>
-        <summary>Gets the root <see cref="T:System.Text.Json.Nodes.JsonNode" />.
-              If the current <see cref="T:System.Text.Json.Nodes.JsonNode" /> is a root, <see langword="null" /> is returned.</summary>
+        <summary>Gets the root <see cref="T:System.Text.Json.Nodes.JsonNode" />.</summary>
         <value>To be added.</value>
         <remarks>
           <format type="text/markdown"><![CDATA[