Custom documents are only used within a given CustomEditorProvider. The lifecycle of a CustomDocument is
managed by the editor. When no more references remain to a CustomDocument, it is disposed of.
This is invoked by the editor when there are no more references to a given CustomDocument (for example when
all editors associated with the document have been closed.)
Represents a custom document used by a
CustomEditorProvider.Custom documents are only used within a given
CustomEditorProvider. The lifecycle of aCustomDocumentis managed by the editor. When no more references remain to aCustomDocument, it is disposed of.