[SPF] Add warning for valid goal constraints

The StatesPathFinder algorithm is not yet able to handle all kinds of
goal constraints. An example is when the object final configuration is
locked in placement: the algorithm is not propagating the placement
complement to previous waypoints. A note is added for users and future
developers.

include/hpp/manipulation/path-planner/states-path-finder.hh

@@ -254,7 +254,13 @@ namespace hpp {
           mutable std::shared_ptr <GraphSearchData> graphData_;
           graph::Edges_t lastBuiltTransitions_;
 
-          // Constraints defining the goal
+          /// Constraints defining the goal
+          /// For now:
+          /// - comparison type Equality is initialized to zero
+          /// - if goal constraint is not already present in any graph state,
+          ///   it should not require propagating a complement.
+          ///   invalid eg: specify the full pose of an object placement or
+          ///   object grasp
           NumericalConstraints_t goalConstraints_;
           bool goalDefinedByConstraints_;
           // Variables used across several calls to oneStep