Grammar fixes for split/merge partitions code
The fixes relate to comments, error messages, and corresponding expected output
of regression tests.
Discussion: https://postgr.es/m/CAMbWs49DDsknxyoycBqiE72VxzL_sYHF6zqL8dSeNehKPJhkKg%40mail.gmail.com
Discussion: https://postgr.es/m/
86bfd241-a58c-479a-9a72-
2c67a02becf8%40postgrespro.ru
Discussion: https://postgr.es/m/CAHewXNkGMPU50QG7V6Q60JGFORfo8LfYO1_GCkCa0VWbmB-fEw%40mail.gmail.com
Author: Richard Guo, Dmitry Koval, Tender Wang