pgsql: Fix outdated bit in README.tuplock

Fix outdated bit in README.tuplock

Apparently this information has been outdated since first committed,
because we adopted a different implementation during development per
reviews and this detail was not updated in the README.

This has been wrong since commit 0ac5ad5134f2 introduced the file in
2013.  Backpatch to all live branches.

Reported-by: Will Mortensen <will@extrahop.com>
Discussion: https://postgr.es/m/CAMpnoC6yEQ=c0Rdq-J7uRedrP7Zo9UMp6VZyP23QMT68n06cvA@mail.gmail.com

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/4a20c83798edc09f4de3a2f833f97ea70f3905d2

Modified Files
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src/backend/access/heap/README.tuplock | 9 ++-------
1 file changed, 2 insertions(+), 7 deletions(-)