pgsql: doc: Clarify that empty COMMENT string removes the comment.

doc: Clarify that empty COMMENT string removes the comment.

Clarify the documentation of COMMENT ON to state that specifying an empty
string is treated as NULL, meaning that the comment is removed.

This makes the behavior explicit and avoids possible confusion about how
empty strings are handled.

Also adds regress test cases that use empty string to remove a comment.

Backpatch to all supported versions.

Author: Chao Li <lic(at)highgo(dot)com>
Reviewed-by: Ashutosh Bapat <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com>
Reviewed-by: David G. Johnston <david(dot)g(dot)johnston(at)gmail(dot)com>
Reviewed-by: Shengbin Zhao <zshengbin91(at)gmail(dot)com>
Reviewed-by: Jim Jones <jim(dot)jones(at)uni-muenster(dot)de>
Reviewed-by: zhangqiang <zhang_qiang81(at)163(dot)com>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Discussion: https://postgr.es/m/26476097-B1C1-4BA8-AA92-0AD0B8EC7190@gmail.com
Backpatch-through: 14

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/7ab6ac1d7f50eb4a36280cbbee3be68bc19d28ca

Modified Files
--------------
doc/src/sgml/ref/comment.sgml | 18 +++++++++++-------
src/test/regress/expected/create_index.out | 14 ++++++++++++++
src/test/regress/sql/create_index.sql | 4 ++++
3 files changed, 29 insertions(+), 7 deletions(-)