Correct JsonNamingPolicy docs (#3244)

This removes the incorrect docs about JsonNamingPolicy being an enum and indicates the behavior if the property is null, as described in dotnet/aspnetcore#10156.

Author: breyed

xml/System.Text.Json/JsonSerializerOptions.xml

@@ -338,8 +338,8 @@ There is a performance cost associated with case-insensitie comparison (that is,
         <ReturnType>System.Text.Json.JsonNamingPolicy</ReturnType>
       </ReturnValue>
       <Docs>
-        <summary>Gets or sets a value that specifies the policy used to convert a property's name on an object to another format, such as camel-casing. </summary>
-        <value>One of the enum values from <see cref="T:System.Text.Json.JsonNamingPolicy" />.</value>
+        <summary>Gets or sets a value that specifies the policy used to convert a property's name on an object to another format, such as camel-casing, or <see langword="null" /> to leave property names unchanged.</summary>
+        <value>A property naming policy, or <see langword="null" /> to leave property names unchanged.</value>
         <remarks>
           <format type="text/markdown"><![CDATA[