pgsql: doc PG 18 relnotes: Add migration note about tsearch

doc PG 18 relnotes: Add migration note about tsearch

Document the small migration hazard introduced in commit fb1a18810f0,
as suggested there.

Reviewed-by: Daniel Verite <daniel@manitou-mail.org>
Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://www.postgresql.org/message-id/flat/653f3b84-fc87-45a7-9a0c-bfb4fcab3e7d%40eisentraut.org

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d1073c3b4cc83bb9d25b0c0203adc74ca58ef866

Modified Files
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doc/src/sgml/release-18.sgml | 22 ++++++++++++++++++++++
1 file changed, 22 insertions(+)