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)

src/backend/replication/walsender.c

@@ -466,6 +466,10 @@ SendTimeLineHistory(TimeLineHistoryCmd *cmd)
 	pq_sendstring(&buf, "content"); /* col name */
 	pq_sendint32(&buf, 0);		/* table oid */
 	pq_sendint16(&buf, 0);		/* attnum */
+	/*
+	 * While this is labeled as BYTEAOID, it is the same output format
+	 * as TEXTOID above.
+	 */
 	pq_sendint32(&buf, BYTEAOID);	/* type oid */
 	pq_sendint16(&buf, -1);		/* typlen */
 	pq_sendint32(&buf, 0);		/* typmod */