You get only one bite at that apple, unfortunately.
The way the Organizer (that part of the galleries that's divided into sections or left as Uncategorized) was set up, you can only add a description when you add a "folder." Once the "folder" has a name and galleries in it, the only things you can do are: change the logo, rename the folder or get rid of it altogether.
All I can suggest is, when you go to create a folder, have a description in mind at the same time. When I set mine up, I had the names, descriptions and logos (avatars?) ready to go. If you're keen on having descriptions to your current folders, the only option I can see is to re-create them and add those descriptions to the new 'versions' as you're doing so. Unfortunately it also means that you'll have to re-assign the galleries that were in the old ones to the new ones, as each folder gets its own number in the system.
Sorry I couldn't be more help.
"He has all of Nelson's qualities, except one. He has not learned to disobey."
- Admiral Sir J.A. Fisher, RN, 1914, referring to Admiral Jellicoe