Add note on the Builder widget's use of the `callback` property.
* `String this.value()` --
A function returning the value of the widget's HTML element.
+* TODO autosearch()
+
+* TODO hideWhenNarrow()
+
+* TODO subwidget()
+
+* TODO expandValue()
+
+In addition to these properties and methods of the bare widget object,
+some kinds of specific widget add other properties of their own. For
+example, the `Builder` widget uses a `callback` property as the
+function that it use to publish the widget definition that it
+constructs. This defaults to the builtin function `alert`, but can be
+overridden by derived widgets such as `ConsoleBuilder`.
+
Team methods
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