update documentation

Extend `numberOfCells.rst`
  - add condition about the absorber
  - mention the parameter `autoAdjustGrid`

docs/TBG_macros.cfg

@@ -53,6 +53,8 @@ TBG_gridSize="128 256 128"
 # Number of simulation steps/iterations as "N"
 TBG_steps="100"
 
+# disable grid size auto adjustment
+TBG_disableGridAutoAdjustment="--autoAdjustGrid off"
 
 ################################################################################
 ## Section: Optional Variables

docs/source/usage/workflows/numberOfCells.rst

@@ -8,10 +8,14 @@ Setting the Number of Cells
 Together with the grid resolution in :ref:`grid.param <usage-params-core>`, the number of cells in our :ref:`.cfg files <usage-tbg>` determine the overall size of a simulation (box).
 The following rules need to be applied when setting the number of cells:
 
-Each GPU needs to:
+Each device needs to:
 
 #. contain an integer *multiple* of supercells
 #. at least *three* supercells
+#. for non periodic boundary conditions, the number of absorbing boundary cells for devices at the simulation boundary (see :ref:`grid.param <usage-params-core>`) must fit into the local volume
+
+The grid size will be automatically adjusted if the conditions above are not fulfilled. 
+This behavior can be disabled by using the command line option ``--autoAdjustGrid off``
 
 Supercell sizes in terms of number of cells are set in :ref:`memory.param <usage-params-memory>` and are by default ``8x8x4`` for 3D3V simulations on GPUs.
 For 2D3V simulations, ``16x16`` is usually a good supercell size, however the default is simply cropped to ``8x8``, so make sure to change it to get more performance.