pgsql: doc: Clarify that COLUMN is optional in ALTER TABLE ... ADD/DROP

doc: Clarify that COLUMN is optional in ALTER TABLE ... ADD/DROP COLUMN.

In ALTER TABLE ... ADD/DROP COLUMN, the COLUMN keyword is optional. However,
part of the documentation could be read as if COLUMN were required, which may
mislead users about the command syntax.

This commit updates the ALTER TABLE documentation to clearly state that
COLUMN is optional for ADD and DROP.

Also this commit adds regression tests covering ALTER TABLE ... ADD/DROP
without the COLUMN keyword.

Backpatch to all supported versions.

Author: Chao Li <lic(at)highgo(dot)com>
Reviewed-by: Robert Treat <rob(at)xzilla(dot)net>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAEoWx2n6ShLMOnjOtf63TjjgGbgiTVT5OMsSOFmbjGb6Xue1Bw@mail.gmail.com
Backpatch-through: 14

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/f51821d9cf769d3f8f400dcef9f46ecd584e1403

Modified Files
--------------
doc/src/sgml/ref/alter_table.sgml | 4 ++--
src/test/regress/expected/alter_table.out | 10 ++++++++++
src/test/regress/sql/alter_table.sql | 8 ++++++++
3 files changed, 20 insertions(+), 2 deletions(-)