lib.types.unspecified: Make name match attribute name again

@Infinisil: This isn't right, the name shouldn't be changed, but instead a description should be added
https://github.com/NixOS/nixpkgs/pull/191353#discussion_r978983401

lib/types.nix

@@ -250,7 +250,8 @@ rec {
     };
 
     unspecified = mkOptionType {
-      name = "unspecified value";
+      name = "unspecified";
+      description = "unspecified value";
       descriptionClass = "noun";
     };