Displaying a list of selected elements in a Document or Dashboard can be easily accomplished using the ConcatAgg function.
Supports multiple element selections.

Step 1 - Metric creation

With the function ConcatAgg, create a metric for the attribute for which you want to get the selection summary.
Here : Category

ConcatAgg([Category@DESC])

Step 2 - Text box

Place the previously created metric in a Text box
 
 

Step 3 - Run your Dashboard or Document

Bonus 

You can specify the delimiter (by default: comma) and the sort order.