Some places still used "Maximum" instead of "Peak" when displaying info
about sort space, so fix that. Also, add a comment clarifying why it's
correct to check the number of full/prefix sort groups.

Author: James Coleman
Discussion: https://postgr.es/m/CAPpHfds1waRZ=NOmueYq0sx1ZSCnt+5QJvizT8ndT2=etZEeAQ@mail.gmail.com