[netinstall] Merge the two descriptions of *immutable*

src/modules/netinstall/netinstall.conf

@@ -162,13 +162,14 @@ label:
 #    Defaults to false. If not set in a subgroup (see below), inherits from
 #    the parent group.
 #  - *immutable*: if true, the state of the group (and all its subgroups)
-#    cannot be changed; it really only makes sense in combination
-#    with *selected* set to true. This only affects the user-interface.
+#    cannot be changed; no packages can be selected or deselected. No
+#    checkboxes are show for the group. Setting *immutable* to true
+#    really only makes sense in combination with *selected* set to true,
+#    so that the packages will be installed. (Setting a group to immutable
+#    can be seen as removing it from the user-interface.)
 #  - *expanded*: if true, the group is shown in an expanded form (that is,
 #    not-collapsed) in the treeview on start. This only affects the user-
 #    interface. Only top-level groups are show expanded-initially.
-#  - *immutable*: if true, the group cannot be changed (packages selected
-#    or deselected) and no checkboxes are shown for the group.
 #  - *subgroups*: if present this follows the same structure as the top level
 #    groups, allowing sub-groups of packages to an arbitary depth.
 #  - *pre-install*: an optional command to run within the new system before