pgsql: Fix outdated bit in README.tuplock

Fix outdated bit in README.tuplock

Apparently this information has been outdated since first committed,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_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/c0ff8261e2b9ba2f78c2b7ed4128f18a260751e0

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