doc: explain hex encoding in Buffer API

fixes: #29786
refs: #29792
refs: #24491

PR-URL: #31352
Fixes: #29786
Refs: #29792
Refs: #24491
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>

doc/api/buffer.md

@@ -203,7 +203,20 @@ The character encodings currently supported by Node.js include:
 
 * `'binary'`: Alias for `'latin1'`.
 
-* `'hex'`: Encode each byte as two hexadecimal characters.
+* `'hex'`: Encode each byte as two hexadecimal characters. Data truncation
+  may occur for unsanitized input. For example:
+
+```js
+Buffer.from('1ag', 'hex');
+// Prints <Buffer 1a>, data truncated when first non-hexadecimal value
+// ('g') encountered.
+
+Buffer.from('1a7g', 'hex');
+// Prints <Buffer 1a>, data truncated when data ends in single digit ('7').
+
+Buffer.from('1634', 'hex');
+// Prints <Buffer 16 34>, all data represented.
+```
 
 Modern Web browsers follow the [WHATWG Encoding Standard][] which aliases
 both `'latin1'` and `'ISO-8859-1'` to `'win-1252'`. This means that while doing