Correct/improve dragbox docs

docs only, no logic change.

Was going to include a reference to the box-selection example #1959, but that hasn't been merged yet :-)
I think that "condition" is not a "conditional modifier". It is a function that gets a map browser event and returns true or false to indicate whether that event should be handled by the interaction. In the case of the drag box interaction you can use for example use the "condition" to indicate that boxes should be drawn only if the shift key is pressed while dragging.