[ci skip] Document millisecond precision change in ActiveSupport::TimeWithZone#as_json

Before, `TimeWithZone#as_json` had no millisecond precision, and now it does. Since this may be a breaking change for some apps or API consumers, we should document this in the Upgrade Gudies.
the original has an extra space at the end of the line, took me a while to figure that out :smile: