Is it possible to add a description to a Gmail label, akin to the tag wikis of Stack Overflow?
Unfortunately you cannot. I don't think it is similar to StackExchange tags as these are used to describe to users not familiar to the tag, what it is about.
With gmail labels, normally there should be one user per account, thus that user should know the reasoning behind a certain tag.
If there is a label you forgot the purpose for, just click it or search
One other option would be to check your labels in
then manually transfer to a private Google docs and leave your information there to remind yourself about certain items pertaining to the label.
Or you could try searching for a user-script that achieves this.
Note that having many labels (100 +) will slow down your activity.
Google just recently changed labels from a maximum of 40 characters to 255 characters, so you can make nice, long, descriptive labels.