pgsql: doc: add reference to wire protocol details

doc: add reference to wire protocol details

Discussion: /messages/by-id/143A51B2-80B1-4ECD-AF67-F7061377FF63@hotmail.com

Author: Li Japin

Backpatch-through: master

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/783e816666c7e70bc79c10aa14dd3d56c61dd483

Modified Files
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doc/src/sgml/protocol.sgml | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)