[skip ci] Add comments about order of post-processing calls

include/engine/api/route_api.hpp

@@ -165,6 +165,10 @@ class RouteAPI : public BaseAPI
                  * to find a via point.
                  * The same exit will be emitted, though, if we should start routing at S, making
                  * the overall response consistent.
+                 *
+                 * ⚠ CAUTION: order of post-processing steps is important
+                 *    - postProcess must be called before collapseTurnInstructions that expects
+                 *      post-processed roundabouts without Exit instructions
                  */
 
                 guidance::trimShortSegments(steps, leg_geometry);

src/engine/guidance/collapse_turns.cpp

@@ -315,6 +315,12 @@ RouteSteps collapseTurnInstructions(RouteSteps steps)
         if (entersRoundabout(current_step->maneuver.instruction) ||
             staysOnRoundabout(current_step->maneuver.instruction))
         {
+            // If postProcess is called before then all corresponding leavesRoundabout steps are
+            // removed and the current roundabout step can be ignored by directly proceeding to
+            // the next step.
+            // If postProcess is not called before then all steps till the next leavesRoundabout
+            // step must be skipped to prevent incorrect roundabouts post-processing.
+
             // are we done for good?
             if (current_step + 1 == steps.end())
                 break;