doc: warn against filling buffer with invalid data

PR-URL: #17428
Backport-PR-URL: #17467
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

Author: addaleax

doc/api/buffer.md

@@ -1253,6 +1253,19 @@ Example: Fill a `Buffer` with a two-byte character
 console.log(Buffer.allocUnsafe(3).fill('\u0222'));
 ```
 
+If `value` is contains invalid characters, it is truncated; if no valid
+fill data remains, no filling is performed:
+
+```js
+const buf = Buffer.allocUnsafe(5);
+// Prints: <Buffer 61 61 61 61 61>
+console.log(buf.fill('a'));
+// Prints: <Buffer aa aa aa aa aa>
+console.log(buf.fill('aazz', 'hex'));
+// Prints: <Buffer aa aa aa aa aa>
+console.log(buf.fill('zz', 'hex'));
+```
+
 ### buf.includes(value[, byteOffset][, encoding])
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