ComboBox|TreeMenu: Clarify :row|column-span-column

ComboBox|TreeMenu: Clarify :row|column-span-column

ComboBox and TreeMenu warned in the doc for :row-span-column that the
value must not exceed :wrap-width, but :wrap-width does not interact
with the number of rows; it’s the :column-span-column that’s relevant.

Also: Warn that spans must be > 0 for rows too, and that column spans <=
:wrap-width are also not useful for items at menu column positions > 0.

Finally, refer to items having spans, not values, as we were already
talking about values in the model (and rows in the menu).