describe
Renders a TrainSetItem as a short label, used wherever an item is named in a log line or a stage name. An identifier is the usual choice.
Keep the label on one line and short: a label is rendered once per item, and a dataset of a few hundred items then fills the log with them. Also, avoid backticks: a log line that embeds a label wraps it in backticks itself.
Defaults to serializeItem; override it when serializeItem renders more than an identifier.