improve doxygen doc.

src/contrib/config-store.h

@@ -8,6 +8,20 @@ namespace ns3 {
 /**
  * \brief Store and load simulation attribute configuration
  *
+ * While it is possible to generate a sample config file and lightly
+ * edit it to change a couple of values, there are cases where this
+ * process will not work because the same value on the same object
+ * can appear multiple times in the same automatically-generated 
+ * configuration file under different configuration paths.
+ *
+ * As such, the best way to use this class is to use it to generate
+ * an initial configuration file, extract from that configuration
+ * file only the strictly necessary elements, and move these minimal
+ * elements to a new configuration file which can then safely
+ * be edited. Another option is to use the ns3::GtkConfigStore class
+ * which will allow you to edit the parameters and will generate 
+ * configuration files where all the instances of the same parameter
+ * are changed.
  */
 class ConfigStore : public ObjectBase
 {