Updating the docs

doc/modules.dox

@@ -298,6 +298,7 @@ These are the toolkit's enumerated types whose members are used as function argu
 \enum EN_RuleVariable
 \enum EN_RuleOperator
 \enum EN_RuleStatus
+\def EN_MISSING
 @}
 */
 

include/epanet2_enums.h

@@ -9,7 +9,7 @@
  Authors:      see AUTHORS
  Copyright:    see AUTHORS
  License:      see LICENSE
- Last Updated: 11/04/2019
+ Last Updated: 11/06/2019
  ******************************************************************************
 */
 
@@ -463,6 +463,6 @@ typedef enum {
   EN_R_IS_ACTIVE = 3    //!< Control valve is active
 } EN_RuleStatus;
 
-#define EN_MISSING -1.E10
+#define EN_MISSING -1.E10  //!< Missing value indicator
 
 #endif //EPANET2_ENUMS_H