Document Cartesian info fields for unstructured grids

The previous text contained an ambiguity of the contents of the field global_cell in case it was non-null; this could be interpreted as if the grid was non-Cartesian. However, this is not the convention that has been followed in current clients.

The new text writes a warning about this, as well as giving a tip on how an unstructured grid may be signaled through the structure (the cartdims field is zero-initialized in the create_empty_grid function, so this array must be actively initialized by the client).