added exceptions to serialize async (#3962)

Author: alanisaac

xml/System.Text.Json/JsonSerializer.xml

@@ -647,6 +647,8 @@ For more information, see [How to serialize and deserialize JSON](~/docs/standar
 
  ]]></format>
         </remarks>
+        <exception cref="T:System.ArgumentNullException">
+          <paramref name="utf8Json" /> or <paramref name="inputType" /> is <see langword="null" />.</exception>
       </Docs>
     </Member>
     <Member MemberName="SerializeAsync&lt;TValue&gt;">
@@ -690,6 +692,8 @@ For more information, see [How to serialize and deserialize JSON](~/docs/standar
 
  ]]></format>
         </remarks>
+        <exception cref="T:System.ArgumentNullException">
+          <paramref name="utf8Json" /> is <see langword="null" />.</exception>
       </Docs>
     </Member>
     <Member MemberName="SerializeToUtf8Bytes">