lib/types: enhances separatedString's description

The previous description "string" is misleading in the full options
manual pages; they are actually concatenated strings, with a specific
character.

The empty string version ("types.string") has been special-cased to
provide a better message.

lib/types.nix

@@ -194,7 +194,10 @@ rec {
     # separator between the values).
     separatedString = sep: mkOptionType rec {
       name = "separatedString";
-      description = "string";
+      description = if sep == ""
+        then "Concatenated string" # for types.string.
+        else "strings concatenated with ${builtins.toJSON sep}"
+      ;
       check = isString;
       merge = loc: defs: concatStringsSep sep (getValues defs);
       functor = (defaultFunctor name) // {