doc: add a note to buf.fill() description

PR-URL: #25547
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/buffer.md

@@ -1235,7 +1235,9 @@ console.log(b.toString());
 // Prints: hhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh
 ```
 
-`value` is coerced to a `uint32` value if it is not a string or integer.
+`value` is coerced to a `uint32` value if it is not a string, `Buffer`, or
+integer. If the resulting integer is greater than `255` (decimal), `buf` will be
+filled with `0`.
 
 If the final write of a `fill()` operation falls on a multi-byte character,
 then only the bytes of that character that fit into `buf` are written: