To handle the edge case, what do you think of this?  Feel free to tweak for clarity.

"""
The global number of elements in an array is the product of the ``size``s of the dimension dictionaries, or 1 if the ``dim_data`` sequence is empty.  In Python syntax, this would be ``reduce(operator.mul, global_shape, 1)`` where ``global_shape`` is a Python sequence of integers such that ``global_shape[i]`` is the ``size`` of the dimension dictionary for dimension ``i``.  If ``global_shape`` is an empty sequence, the result of the reduction above is ``1``, indicating the distributed array is a zero-dimensional scalar.
"""