doc: demonstrate dangers of buffer.slice()

PR-URL: nodejs#41628
Reviewed-By: Benjamin Gruenbaum <[email protected]>
Reviewed-By: Antoine du Hamel <[email protected]>
Reviewed-By: Evan Lucas <[email protected]>
Reviewed-By: Mohammed Keyvanzadeh <[email protected]>
Reviewed-By: Mestery <[email protected]>
Reviewed-By: Rich Trott <[email protected]>

Author: shalvah

doc/api/buffer.md

@@ -3427,6 +3427,14 @@ 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
@@ -3441,6 +3449,14 @@ 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()`