IgnoreNullValues isn't for deserialization (dotnet#5470)

Author: tdykstra

xml/System.Text.Json/JsonSerializerOptions.xml

@@ -356,19 +356,21 @@ For more information, see [How to write custom converters for JSON serialization
         <ReturnType>System.Boolean</ReturnType>
       </ReturnValue>
       <Docs>
-        <summary>Gets or sets a value that determines whether <see langword="null" /> values are ignored during serialization and deserialization. The default value is <see langword="false" />.</summary>
+        <summary>Gets or sets a value that determines whether <see langword="null" /> values are ignored during serialization. The default value is <see langword="false" />.</summary>
         <value>
-          <see langword="true" /> to ignore null values during serialization and deserialization; otherwise, <see langword="false" />.</value>
+          <see langword="true" /> to ignore null values during serialization; otherwise, <see langword="false" />.</value>
         <remarks>
           <format type="text/markdown"><![CDATA[
 
 ## Remarks
 
+This setting doesn't apply to deserialization.
+
 For more information, see [How to ignore properties](/dotnet/standard/serialization/system-text-json-ignore-properties).
 
  ]]></format>
         </remarks>
-        <exception cref="T:System.InvalidOperationException">This property was set after serialization or deserialization has occurred.
+        <exception cref="T:System.InvalidOperationException">This property was set after serialization has occurred.
 
 -or-