This event is emitted when certain user actions are performed to an annotation. See PSPDFKit.Instance~AnnotationsWillChangeEvent
A list of affected annotations. At the moment, this can contain at most a single annotation.
In some cases, it can return an empty list. Please See PSPDFKit.AnnotationsWillChangeReason for a complete reference when this is the case.
This indicates the reason why the annotation will change.