For obsolote method alternate suggesting XmlDateTimeSerializationMode

[buyaa-n]

Summary

Method XmlConvert.ToString(DateTime) is obsolete, and suggested alternate is XmlConvert.ToString(DateTime, XmlDateTimeSerializationMode) customers want to know what XmlDateTimeSerializationMode is best fit for the replacement. Suggested mode is XmlDateTimeSerializationMode.RoundtripKind but it will not produce exact matching result in some conditions, and there is no other matching mode which produce exact matching replacement, in that case ToString(DateTime, String) can be used with format string "yyyy-MM-ddTHH:mm:ss.fffffffzzzzzz". Updating notes section for suggested XmlDateTimeSerializationMode and exact match atlernate

Fixes #2665

[tarekgh]

LGTM. can we also suggest to use the overload which takes DateTimeOffset too?

[carlossanlop]

Thanks for your PR, @buyaa-n. I left you some suggested changes for you to consider.

[buyaa-n]

LGTM. can we also suggest to use the overload which takes DateTimeOffset too?

Not sure, for using ToString(DateTimeOffset, String) overload current DateTime instance need to be converted or cast to DateTimeOffset type, cannot be used directly i assume. Then next question would come up like which conversion would give closest/exact match etc, not sure if we need to add such info in this doc

[carlossanlop]

Thank you @buyaa-n for applying the suggestions. The build has no warnings or failures, so it's good to merge.

In case you and @tarekgh decide to mention his suggestion, feel free to add it in a separate PR.