buffer: document concat zero-fill

PR-URL: #55562
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Ulises Gascón <ulisesgascongonzalez@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jason Zhang <xzha4350@gmail.com>

doc/api/buffer.md

@@ -1042,7 +1042,8 @@ in `list` by adding their lengths.
 
 If `totalLength` is provided, it is coerced to an unsigned integer. If the
 combined length of the `Buffer`s in `list` exceeds `totalLength`, the result is
-truncated to `totalLength`.
+truncated to `totalLength`. If the combined length of the `Buffer`s in `list` is
+less than `totalLength`, the remaining space is filled with zeros.
 
 ```mjs
 import { Buffer } from 'node:buffer';