It seems not possible to add a second row in the table header in App Designer. Also, when you create a table in app designer. It gets created under uifigure parent and column names are restricted to one line of text. Therefore, you cannot add a second line for the units / examples also.
But when you create a table directly without specifying any parent (using the uitable() function) then default parent is a MATLAB figure and the uitable is created for which column name property supports "|" to get the multi-line column name, where the second line can be used for the units / examples.