Replace "transaction log" with "write-ahead log"

This makes documentation and error messages match the renaming of "xlog"
to "wal" in APIs and file naming.

src/backend/postmaster/checkpointer.c

@@ -626,9 +626,8 @@ CheckArchiveTimeout(void)
 			 * assume nothing happened.
 			 */
 			if ((switchpoint % XLogSegSize) != 0)
-				ereport(DEBUG1,
-						(errmsg("transaction log switch forced (archive_timeout=%d)",
-								XLogArchiveTimeout)));
+				elog(DEBUG1, "write-ahead log switch forced (archive_timeout=%d)",
+					 XLogArchiveTimeout);
 		}
 
 		/*

src/backend/postmaster/pgarch.c

@@ -482,7 +482,7 @@ pgarch_ArchiverCopyLoop(void)
 				if (++failures >= NUM_ARCHIVE_RETRIES)
 				{
 					ereport(WARNING,
-							(errmsg("archiving transaction log file \"%s\" failed too many times, will try again later",
+							(errmsg("archiving write-ahead log file \"%s\" failed too many times, will try again later",
 									xlog)));
 					return;		/* give up archiving for now */
 				}
@@ -628,8 +628,7 @@ pgarch_archiveXlog(char *xlog)
 
 		return false;
 	}
-	ereport(DEBUG1,
-			(errmsg("archived transaction log file \"%s\"", xlog)));
+	elog(DEBUG1, "archived write-ahead log file \"%s\"", xlog);
 
 	snprintf(activitymsg, sizeof(activitymsg), "last was %s", xlog);
 	set_ps_display(activitymsg, false);

src/backend/postmaster/postmaster.c

@@ -5036,7 +5036,7 @@ sigusr1_handler(SIGNAL_ARGS)
 	{
 		/*
 		 * Send SIGUSR1 to archiver process, to wake it up and begin archiving
-		 * next transaction log file.
+		 * next WAL file.
 		 */
 		signal_child(PgArchPID, SIGUSR1);
 	}