pgsql: doc: clarify wording about phantom reads

doc: clarify wording about phantom reads

Reported-by: akhilhello@gmail.com

Discussion: /messages/by-id/165222922369.669.10475917322916060899@wrigleys.postgresql.org

Backpatch-through: 10

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/3485f8d03eb8cdaa1100472fd36761b1a4b82337

Modified Files
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doc/src/sgml/high-availability.sgml | 2 +-
doc/src/sgml/mvcc.sgml | 7 ++++---
2 files changed, 5 insertions(+), 4 deletions(-)