add c-grid todo comment

Author: reint-fischer

docs/user_guide/examples/tutorial_interpolation.ipynb

@@ -178,7 +178,11 @@
    "source": [
     "The white box is here the 'land' point where the tracer is set to zero and the crosses are the locations of the grid points. As you see, the interpolated value is always equal to the field value if the particle is exactly on the grid point (circles on crosses).\n",
     "\n",
-    "For `interp_method='XNearest'`, the particle values are the same for all particles in a grid cell. They are also the same for `interp_method='CGrid_Tracer'`, but the grid cells have then shifted. That is because in a C-grid, the tracer grid cell is on the top-right corner (black dashed lines in right-most panel).\n",
+    "For `interp_method='XNearest'`, the particle values are the same for all particles in a grid cell. They are also the same for `interp_method='CGrid_Tracer'`, but the grid cells have then shifted. That is because in a C-grid, the tracer is defined on the corners of the velocity grid (black dashed lines in right-most panel).\n",
+    "\n",
+    "```{note}\n",
+    "TODO: check C-grid indexing after implementation in v4\n",
+    "```\n",
     "\n",
     "For `interp_method='XLinearInvdistLandTracer'`, we see that values are the same as `interp_method='XLinear'` for grid cells that don't border the land point. For grid cells that do border the land cell, the `XLinearInvdistLandTracer` interpolation method gives higher values, as also shown in the difference plot below.\n"
    ]