For obsolote method alternate suggesting XmlDateTimeSerializationMode (#3936)

* For obsolote method alternate suggesting XmlDateTimeSerializationMode can be used and exact replacement suggestion

* Update xml/System.Xml/XmlConvert.xml

Co-Authored-By: Carlos Sanchez Lopez <[email protected]>

* Add paragrapgh

Co-authored-by: Carlos Sanchez Lopez <[email protected]>

Author: buyaa-n

xml/System.Xml/XmlConvert.xml

@@ -1844,6 +1844,7 @@ writer.WriteElementString(XmlConvert.EncodeName("Order Detail"),"My order");
   
 > [!NOTE]
 >  The <xref:System.Xml.XmlConvert.ToString%28System.DateTime%29?displayProperty=nameWithType> method is obsolete in the 2.0 version of the .NET Framework and has been replaced with the <xref:System.Xml.XmlConvert.ToString%28System.DateTime%2CSystem.Xml.XmlDateTimeSerializationMode%29?displayProperty=nameWithType> method.  
+>  The suggested mode is <xref:System.Xml.XmlDateTimeSerializationMode.RoundtripKind>. If an exact match is expected, please use <xref:System.Xml.XmlConvert.ToString%28System.DateTime%2CSystem.String%29?displayProperty=nameWithType> with the format string `yyyy-MM-ddTHH:mm:ss.fffffffzzzzzz`. 
   
  ]]></format>
         </remarks>