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)

doc/src/sgml/protocol.sgml

@@ -1425,7 +1425,10 @@ The commands accepted in walsender mode are:
      <para>
       Requests the server to send over the timeline history file for timeline
       <replaceable class="parameter">tli</replaceable>.  Server replies with a
-      result set of a single row, containing two fields:
+      result set of a single row, containing two fields.  While the
+      fields are labeled as <type>text</type> and <type>bytea</type>,
+      they effectively return raw bytes, with no escaping or encoding
+      conversion:
      </para>
 
      <para>

src/backend/replication/walsender.c

@@ -462,6 +462,10 @@ SendTimeLineHistory(TimeLineHistoryCmd *cmd)
 	pq_sendstring(&buf, "content");		/* col name */
 	pq_sendint(&buf, 0, 4);		/* table oid */
 	pq_sendint(&buf, 0, 2);		/* attnum */
+	/*
+	 * While this is labeled as BYTEAOID, it is the same output format
+	 * as TEXTOID above.
+	 */
 	pq_sendint(&buf, BYTEAOID, 4);		/* type oid */
 	pq_sendint(&buf, -1, 2);	/* typlen */
 	pq_sendint(&buf, 0, 4);		/* typmod */