Add comment on Birks' Law usage in SDAction

Add comment about using our custom Birks' Law implementation in the
SDAction. In particular to avoid potential double application of Birks'
correction if the Geant4StepHandler is used on this subdetector.

plugins/DRTubesSDAction.cpp

@@ -109,6 +109,13 @@ bool Geant4SensitiveAction<DRTubesSDData>::process(const G4Step* aStep,
   // (see https://github.com/AIDASoft/DD4hep/issues/1319).
   // Therefore we use copynumbers instead of volIDs.
 
+  // NOTE: in this SDAction we apply our custom Birks' Law correction
+  // on the G4Step via the method DRTubesSglHpr::ApplyBirks().
+  // However it should be known that the dd4hep Geant4StepHandler can apply Birks' Law
+  // correction too (using internally the Geant4 class G4EmSaturation).
+  // Therefore, if the Geant4StepHandler is used on this subdetector it might
+  // lead to a double application of Birks' correction.
+
 #ifdef DRTubesSDDebug
   // Print out some info step-by-step in sensitive volumes
   //