Documentation Request: How to add hidden fields

[rickspencer3]

The user would like to add a "date created" property to each document, but does not want that to appear in the UI or be editable.

Their options are:

1. DocumentList automatically creates or updates a _time_stamp with each edit of the document. This changes, but if last edit date is sufficient, this property should be exposed.
2. The user could respond to the onChanged event of the DocumentList, find the last item, and add the creation date then.
3. Any field in the document that starts with "_" is not displayed, so they could add "_creation_date" as a field.
4. They could supply a labels list that does not include the creation date.

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We should simplify this scenario by adding an "onDocumentCreated(Map<String, dynamic> document, int index){/do something with the document/} event.

We should document that these fields are created and different ways to use them.

We should document how "_" works.