Update documentation of (clear|reset)ConfigValidations.

* https://github.com/humanoid-path-planner/hpp-core/issues/136

include/hpp/core/problem.hh

@@ -139,9 +139,11 @@ namespace hpp {
       }
 
       /// Reset the ConfigValidations
+      /// by creating a new ConfigValidations using ConfigValidations::create
       void resetConfigValidations ();
 
-      // Clear the ConfigValidations
+      /// Clear the ConfigValidations
+      /// by calling ConfigValidations::clear
       void clearConfigValidations ();
 
       /// Add a config validation method