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
------
REL_12_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/c08b3e9413b745b005c67b5333569055066a6a9e

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