pgsql: Grammar fixes for split/merge partitions code

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9dfcac8e15acc3b4d4d5bc02383a56ccb07229fe

Modified Files
--------------
doc/src/sgml/ddl.sgml                         |  2 +-
doc/src/sgml/ref/alter_table.sgml             |  6 +--
src/backend/commands/tablecmds.c              | 21 +++++-----
src/backend/parser/parse_utilcmd.c            |  6 +--
src/backend/partitioning/partbounds.c         | 35 ++++++++--------
src/test/isolation/specs/partition-merge.spec |  2 +-
src/test/regress/expected/partition_merge.out | 14 +++----
src/test/regress/expected/partition_split.out | 58 +++++++++++++--------------
src/test/regress/sql/partition_merge.sql      | 12 +++---
src/test/regress/sql/partition_split.sql      | 48 +++++++++++-----------
10 files changed, 104 insertions(+), 100 deletions(-)