qgscameracontroller: Add comments for MouseOperation enum

src/3d/qgscameracontroller.h

@@ -225,14 +225,15 @@ class _3D_EXPORT QgsCameraController : public QObject
     //! Returns a pointer to the scene's engine's window or nullptr if engine is QgsOffscreen3DEngine
     QWindow *window() const;
 
+    //! List of possible operations with the mouse in TerrainBased navigation
     enum class MouseOperation
     {
-      None = 0,
-      Translation,
-      RotationCamera,
-      RotationCenter,
-      Zoom,
-      ZoomWheel
+      None = 0,       // no operation
+      Translation,    // left button pressed, no modifier
+      RotationCamera, // left button pressed + ctrl modifier
+      RotationCenter, // left button pressed + shift modifier
+      Zoom,           // right button pressed
+      ZoomWheel       // mouse wheel scroll
     };
 
     // This list gathers all the rotation and translation operations.