Add more info about the linked group feature.

src/gui/auxiliary/window_builder.cpp

@@ -273,6 +273,18 @@ twindow_builder::tresolution::tresolution(const config& cfg) :
  *
  *     grid (grid)                   The grid with the widgets to show.
  * @end_table
+ *
+ * A linked_group section has the following fields:
+ * @start_table = config
+ *     id (string)                   The unique id of the group (unique in this
+ *                                   window).
+ *     fixed_width (bool = false)    Should widget in this group have the same
+ *                                   width.
+ *     fixed_height (bool = false)   Should widget in this group have the same
+ *                                   height.
+ * @end_table
+ *
+ * A linked group needs to have at least one size fixed.
  */
 
 	const config &c = cfg.child("grid");

src/gui/auxiliary/window_builder/control.cpp

@@ -112,6 +112,8 @@ void tbuilder_control::init_control(tcontrol* control) const
  *                                     the tooltip is empty the label can be
  *                                     used for the tooltip. If this variable is
  *                                     set to true this will happen.
+ *
+ *   linked_group (string = "")        The linked group the widget belongs to.
  * @end_table
  *
  */

src/gui/auxiliary/window_builder/listbox.cpp

@@ -156,6 +156,8 @@ twidget* tbuilder_listbox::build() const
  *
  * @end_table
  *
+ * In order to force widgets to be the same size inside a listbox, the widgets
+ * need to be inside a linked_group.
  *
  * Inside the list section there are only the following widgets allowed
  * * grid (to nest)