No, there is currently no way to add description to possible values of URI parameters.
Neither
+ Values
+ `A - means something`
+ `B`
+ `C`
or
+ Values
+ `A` means something
+ `B`
+ `C`
will work correctly. I filed a feature request under API Blueprint's repository. If you want to be part of the design process and help us to get the best solution to your problem, you can track it and comment under it.
Using tables
When in troubles with API Blueprint, you can always use plain old Markdown in endpoint's description to supplement or substitute what's missing. E.g. you can freely use tables as an addition or replacement to the Values
section:
# My API
## Sample [/endpoint{?id}]
Description.
| Value | Meaning |
| ------------ |:----------------:|
| A | Alaska |
| B | Bali |
| C | Czech Republic |
+ Parameters
+ id (string)
Description...
| Value | Meaning |
| ------------ |:----------------:|
| A | Alaska |
| B | Bali |
| C | Czech Republic |
Description...
+ Values
+ `A`
+ `B`
+ `C`