Task may add a custom undoable edit related to its operation by using this method.
When undo would be performed after the save then this edit would be undone
(together with any possible modification changes performed by the task on the underlying document).
Note: this method should only be called during OnSaveTask.performTask().
edit - a custom undoable edit provided by the task.
Get a root element with zero or more child elements each designating a modified region
of a document.
Tasks may use this information to work on modified document parts only.
Note: unlike in some other root element implementations here the child elements
do not fully cover the root element's offset space.
root element containing modified regions of document as child elements.
Null if document's implementation does not support modified elements collecting.