pgsql: doc: PG 16 relnotes: clarify "relation" segsize mention

doc: PG 16 relnotes: clarify "relation" segsize mention

Reported-by: harukat@sraoss.co.jp

Discussion: /messages/by-id/18124-d363fa4873e176d6@postgresql.org

Backpatch-through: 16 only

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e391939b76f941544fdb512b147950b71e8ee358

Modified Files
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doc/src/sgml/release-16.sgml | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)