pgsql: Doc: fix stale text about partition locking with cached plans

Doc: fix stale text about partition locking with cached plans

Commit 121d774caea added text to master describing pruning-aware
locking behavior introduced by 525392d57.  That behavior was
reverted in May 2025, making the text incorrect.  Replace it with
the text used in back branches, which correctly describes current
behavior: pruned partitions are still locked at the beginning of
execution.

Discussion: https://postgr.es/m/CA+HiwqFT0fPPoYBr0iUFWNB-Og7bEXB9hB=6ogk_qD9=OM8Vbw@mail.gmail.com

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/a1baf64589758ed488eb943d4ccb64f23c0500ef

Modified Files
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doc/src/sgml/ddl.sgml | 10 +++-------
1 file changed, 3 insertions(+), 7 deletions(-)