pgsql: doc: wire protocol data type for history file content is bytea

doc:  wire protocol data type for history file content is bytea

Document that though the history file content is marked as bytea, it is
the same a text, and neither is btyea-escaped or encoding converted.

Reported-by: Brar Piening

Discussion: https://postgr.es/m/6a1b9cd9-17e3-df67-be55-86102af6bdf5@gmx.de

Backpatch-through: 13 - 9.5 (not master)

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/fe904ac20bd042edf05cc450541cdcd811d4137d

Modified Files
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doc/src/sgml/protocol.sgml          | 5 ++++-
src/backend/replication/walsender.c | 4 ++++
2 files changed, 8 insertions(+), 1 deletion(-)