document max_finite_coordinate

[Numero7]

          They seem to pass now, but you should go over this at some point:

What is the "max_finite_coordinate" argument in DownSet.safe_pre_image?
it is not documented in the docstring above and I could not figure out what it should be.
The doctests appear to be just copies of the unit tests in this module.

Originally posted by @pazz in #89 (comment)

[Numero7]

@pazz thanks, I will take care of documenting this parameter.

It is used in order to heuristically speed-up the search for positive witnesses with small finite constants.