[users] Expand the example config with new possibilities

src/modules/users/users.conf

@@ -16,13 +16,29 @@
 ---
 # Used as default groups for the created user.
 # Adjust to your Distribution defaults.
+#
+# Each entry in the *defaultGroups* list is either:
+#   - a string, naming a group; this is a **non**-system group
+#     which does not need to exist in the target system; if it
+#     does not exist, it will be created.
+#   - an entry with subkeys *name*, *must_exist* and *system*;
+#     if the group *must_exist* and does not, an error is thrown
+#     and the installation fails.
+#
+#     The group is created if it does not exist, and it is
+#     created as a system group (GID < 1000) or user group
+#     (GID >= 1000) depending on the value of *system*.
 defaultGroups:
-    - users
+    - name: users
+      must_exist: true
+      system: true
     - lp
     - video
     - network
     - storage
-    - wheel
+    - name: wheel
+      must_exist: false
+      system: true
     - audio
 
 # Some Distributions require a 'autologin' group for the user.