doc: demonstrate dangers of buffer.slice()

PR-URL: #41628
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Mohammed Keyvanzadeh <mohammadkeyvanzade94@gmail.com>
Reviewed-By: Mestery <mestery@protonmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

doc/api/buffer.md

@@ -3409,6 +3409,14 @@ console.log(copiedBuf.toString());
 
 console.log(buf.toString());
 // Prints: buffer
+
+// With buf.slice(), the original buffer is modified.
+const notReallyCopiedBuf = buf.slice();
+notReallyCopiedBuf[0]++;
+console.log(notReallyCopiedBuf.toString());
+// Prints: cuffer
+console.log(buf.toString());
+// Also prints: cuffer (!)
 ```
 
 ```cjs
@@ -3423,6 +3431,14 @@ console.log(copiedBuf.toString());
 
 console.log(buf.toString());
 // Prints: buffer
+
+// With buf.slice(), the original buffer is modified.
+const notReallyCopiedBuf = buf.slice();
+notReallyCopiedBuf[0]++;
+console.log(notReallyCopiedBuf.toString());
+// Prints: cuffer
+console.log(buf.toString());
+// Also prints: cuffer (!)
 ```
 
 ### `buf.swap16()`