pgsql: Minor improvements in Incremental Sort explain

Minor improvements in Incremental Sort explain

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d22782a5392f6a1cb6cfca34031b93eb9dd2aa03

Modified Files
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src/backend/commands/explain.c                 |  20 +++-
src/test/regress/expected/incremental_sort.out | 134 ++++++++++++-------------
src/test/regress/sql/incremental_sort.sql      |   4 +-
3 files changed, 87 insertions(+), 71 deletions(-)