Incorrect Voronoi bounds from `DimensionCoordinate.create_bounds` for cyclic coordinates

[davidhassell]

At version 3.13.1, incorrect Voronoi bounds are created from DimensionCoordinate.create_bounds for cyclic and period coordinates.

>>> import cf
>>> cf.__version__
'3.13.1'
>>> d = cf.DimensionCoordinate(data=[180, 90, 40, 0.0])   
>>> d.period(360)
>>> d.cyclic(0)
set()
>>> b = d.create_bounds()
>>> print(b.array) 
[[225. 135.]   # Wrong
 [135.  65.]
 [ 65.  20.]
 [ 20. -20.]]  # Wrong

when the result should be

[[270. 135.]   # Correct
 [135.  65.]
 [ 65.  20.]
 [ 20. -90.]]  # Correct

Also, the docstring for the method is lacking in detail.