OBJImporter: Document default behaviors

IO/Import/vtkOBJImporter.h

@@ -63,6 +63,12 @@ public:
   ///@{
   /**
    * Specify the name of the file to read.
+   * FileName must be provided.
+   * FileNameMTL can be provided, if not provided, we will do, in order:
+   *  - Use mtllib is provided in the .obj file
+   *  - Check for a FileName.mtl and use it if it exists
+   *  - Check for a FileStem.mtl and use it if it exists
+   * TexturePath can be provided, it not provided, the folder containing FileName will be used
    */
   void SetFileName(VTK_FILEPATH const char* arg);
   void SetFileNameMTL(VTK_FILEPATH const char* arg);