Moves the given
iter backward to the next display line start.
A display line is different from a paragraph. Paragraphs are
separated by newlines or other paragraph separator characters.
Display lines are created by line-wrapping a paragraph. If
wrapping is turned off, display lines and paragraphs will be the
same. Display lines are divided differently for each view, since
they depend on the view’s width; paragraphs are the same in all
views, since they depend on the contents of the
The data is owned by the caller of the method.