Add note about trailing second fraction zeros being omitted when writing DateTime[Offset]s in System.Text.Json

[layomia]

Issue description

The section about how the serializer and writer produce these types doesn't provide this information: https://docs.microsoft.com/en-us/dotnet/standard/datetime/system-text-json-support#support-for-formatting.

This test data set shows how the omission is done.

[ahsonkhan]

I am assigning this to you @layomia since you wrote the doc. If you think this would be good for someone in the community to fix up, please mark is as up for grabs.