doc: Buffer.toString(): add note about invalid data

PR-URL: #30706
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>

doc/api/buffer.md

@@ -2162,6 +2162,8 @@ added: v0.1.90
 
 Decodes `buf` to a string according to the specified character encoding in
 `encoding`. `start` and `end` may be passed to decode only a subset of `buf`.
+If a byte sequence in the input is not valid in the given `encoding` then
+it is replaced with the replacement character `U+FFFD`.
 
 The maximum length of a string instance (in UTF-16 code units) is available
 as [`buffer.constants.MAX_STRING_LENGTH`][].