Commit 121d774cae added text to master describing pruning-aware
locking behavior introduced by 525392d57. That behavior was
reverted in May 2025, making the text incorrect. Replace it with
the text used in back branches, which correctly describes current
behavior: pruned partitions are still locked at the beginning of
execution.
Discussion: https://postgr.es/m/CA+HiwqFT0fPPoYBr0iUFWNB-Og7bEXB9hB=6ogk_qD9=OM8Vbw@mail.gmail.com