Update JsonSerializerOptions.xml

Refine IncludeFields remarks to highlight that only public
fields are serialized and that private fields are not.

Author: RickDMyers

xml/System.Text.Json/JsonSerializerOptions.xml

@@ -710,8 +710,10 @@ For more information, see [How to ignore properties with System.Text.Json](/dotn
         <summary>Gets or sets a value that indicates whether fields are handled during serialization and deserialization.
             The default value is <see langword="false" />.</summary>
         <value>
-          <see langword="true" /> if fields are included during serialization; otherwise, <see langword="false" />.</value>
-        <remarks>To be added.</remarks>
+          <see langword="true" /> if public fields are included during serialization; otherwise, <see langword="false" />.</value>
+        <remarks>Only includes public fields during serialization and deserialization. Private fields are not included, even if IncludeFields is set to <see langword="true" />.
+                 If you need to include private fields, you would typically use properties with appropriate accessors or consider using attributes like [JsonInclude] on those fields.
+        </remarks>
         <exception cref="T:System.InvalidOperationException">This property is set after serialization or deserialization has occurred.</exception>
       </Docs>
     </Member>