Simple Label field in Document Type Editor for informative display when editing a document

At the moment, we cannot add a simple label in document type editor.
"Hint" might be useful only for a specific field. Sometimes, however, we want to put some more descriptive information for content authors anywhere in a document. For example, we could give more instruction before starting a content block section.
It would be great if we can add a simple label field which is not associated with any physical properties or node, but which is only for display purpose when editing a document.