documentation for binary_piecewise model

[scottransom]

The binary_piecewise model has been in PINT since #1545 and has now been used in at least two papers on the arXiv:
https://ui.adsabs.harvard.edu/abs/2024arXiv241208688C/abstract
https://ui.adsabs.harvard.edu/abs/2024arXiv241021648R/abstract

The referee for one of them has complained that the model is not documented in the PINT docs at all. It would be great if @poneill129 would be able to write up just a couple paragraphs that described the model and the various paramters. (Or if not Patrick, then some other enterprising individual!)

Scott