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_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/921f4a15c10f9791d6d57de7a4e714ef2d3a5fa9

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(-)