[doc] Add a comment in notes.

doc/notes

@@ -24,6 +24,8 @@ In include/hpp/fcl/traversal:
     - bool initialize(MeshCollisionTraversalNodekIOS& node, ...
     - bool initialize(MeshCollisionTraversalNodeOBBRSS& node, ...
 
+-------------------------------------------------------------------------------
+
 classes
   - MeshCollisionTraversalNodeRSS,
   - MeshCollisionTraversalNodekIOS,
@@ -35,3 +37,11 @@ derive from
 They store the relative position and orientation between two objects to test for
 collision. before calling overlap function, this additional information computes
 the relative positions of the bounding volumes.
+
+-------------------------------------------------------------------------------
+
+For primitive shapes, collision and distance computation are very close.
+ShapeShapeCollide calls indeed ShapeShapeDistance. It would be convenient to
+merge CollisionRequest and DistanceRequest on the one hand and CollisionResult
+and DistanceResult on the other hand into two classes InteractionRequest and
+InteractionResult.
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