pgsql: doc: warn about the use of "ctid" queries beyond the examples

doc: warn about the use of "ctid" queries beyond the examples

Also be more assertive that "ctid" should not be used for long-term
storage.

Reported-by: Bernice Southey

Discussion: /messages/by-id/CAEDh4nyn5swFYuSfcnGAbpQrKOc47Hh_ZyKVSPYJcu2P=51Luw@mail.gmail.com

Backpatch-through: 17

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/c6d2cd06cb43050fbe5cc1a928bdb9eb0299ca27

Modified Files
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doc/src/sgml/ddl.sgml | 2 +-
doc/src/sgml/ref/delete.sgml | 3 +++
doc/src/sgml/ref/update.sgml | 3 +++
3 files changed, 7 insertions(+), 1 deletion(-)