pgsql: Improve the protocol message descriptions for 2PC logical replic

Improve the protocol message descriptions for 2PC logical replication.

The messages were using 'two-phase transaction' at some places and
'prepared transaction' at other places. Make them consistently use
'prepared transaction'.

Reported-by: Ekaterina Kiryanova
Author: Peter Smith
Reviewed by: Amit Kapila
Backpatch-through: 15
Discussion: https://postgr.es/m/745414e7-efb2-a6ae-5b83-fcbdf35aabc8@postgrespro.ru

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/c5d67881d343a507269bde124a49df19e0296157

Modified Files
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doc/src/sgml/protocol.sgml | 28 ++++++++++++++--------------
1 file changed, 14 insertions(+), 14 deletions(-)