20.420 text:min-label-distance - improve/clarify/correct specification text

Replace "The text:min-label-distance attribute specifies the minimum distance between a number and a list item." by "The text:min-label-distance attribute specifies the minimum distance between the list label and the text of the list item. [new paragraph] The content of a list label is rendered inside a specified minimum width - see text:min-label-width. The text of a list item is rendered following the label width area. If the distance between a list label and the text of a list item is smaller than the value of the text:min-label-distance attribute, the following actions are performed. First, the content of the list label is moved inside the specified minimum label width up to the value of the text:min-label-distance attribute. If that does not result in a value equal to the text:min-label-distance attribute, the text of the list item is moved until the distance between the list label and text of list item equals the value of the text:min-label-distance attribute."