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).
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