In the description field of the footprint 
But yes some of them have a rather basic description. And the description is not listed when selecting a footprint.
But if you use the method where you use the footprint browser to select a footprint you get a preview. (The description is still nowhere listed. Why do we even bother to fill it out? Well hopefully in the future something nice will be done that shows the description somewhere.)
Here a better explanation about the footprint selection option with a preview. (works in current stable.)
First open the properties field of the symbol you want to connect to a footprint. (press “e” while your mouse pointer is above the symbol.)
Select the footprint field and press the browse footprint button.
Then the footprint browser opens. (In the browser you need a basic idea where what footprints might live. I don’t know of a search field in there.)
I hope this helps at least a bit.