Updated cursor documentation

Hi, thanks for Fractal; it's been a big help in one of my projects. I was reading the documentation online and noticed that the argument for previous cursor was not present; I feel that this may confuse people until they look at the implementation of the default constructor in the Cursor class. I am submitting a small change to the documentation to explicitly show that there is an option for a previous cursor value. Please let me know if there are any thoughts; I am new to Github and git so I am sorry if I have done something incorrectly.