@dpc There’s no documentation (regarding your issue with browsersync) because config.json uses a standard format for configuring your packages - this has already been explained. In this case, the issue you’ve referenced was actually a bug, and has already been fixed, though @drawcardau 's solution would also work.
Bear in mind Sage 9 is undergoing very rapid development, so any issues are likely to not going to be current even days after posting them.