Fix code comments still referring to pg_start/stop_backup()

pg_start_backup() and pg_stop_backup() have been respectively renamed to
pg_backup_start() and pg_backup_stop() as of 39969e2, but a few comments
did not get the call.

Reviewed-by: Kyotaro Horiguchi, David Steele
Discussion: https://postgr.es/m/YrqGlj1+4DF3dbZ/@paquier.xyz

src/backend/access/transam/xlogrecovery.c

@@ -716,7 +716,7 @@ InitWalRecovery(ControlFileData *ControlFile, bool *wasShutdown_ptr,
 		 * know how far we need to replay the WAL before we reach consistency.
 		 * This can happen for example if a base backup is taken from a
 		 * running server using an atomic filesystem snapshot, without calling
-		 * pg_start/stop_backup. Or if you just kill a running primary server
+		 * pg_backup_start/stop. Or if you just kill a running primary server
 		 * and put it into archive recovery by creating a recovery signal
 		 * file.
 		 *

src/backend/replication/basebackup.c

@@ -16,7 +16,7 @@
 #include <unistd.h>
 #include <time.h>
 
-#include "access/xlog_internal.h"	/* for pg_start/stop_backup */
+#include "access/xlog_internal.h"	/* for pg_backup_start/stop */
 #include "common/compression.h"
 #include "common/file_perm.h"
 #include "commands/defrem.h"