20.421 text:min-label-width - improve/clarify/correct specification text

replace "The text:min-label-width attribute specifies the minimum width of a number." by "The text:min-label-width attribute specifies the minimum width of a list label." replace "The label can be aligned horizontally with the width using an fo:text-align attribute." by "The list label can be aligned horizontally within the specified minimum width using the fo:text-align attribute. If the the actual width of the list label is greater than the specified minimum width no alignment takes place."