Negative values never disabled the pruning - they simply set high values for the pruning interval.
Once https://reviews.llvm.org/D41231 lands the behaviour will be that negative values set the maximum pruning interval (which appears to have been the intention from the start).
I have adjusted the comments to reflect this, removed any inaccurate statements, and corrected any typos I spotted in the English.
I prefer leaving a note here about the value -1. You can note that it sets the largest possible pruning interval, effectively disabling it.