doc: add fs.open() multiple constants example

PR-URL: nodejs/node#33281
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

Author: Ethan-Arrowood

doc/api/fs.md

@@ -5434,6 +5434,24 @@ The following constants are exported by `fs.constants`.
 
 Not every constant will be available on every operating system.
 
+To use more than one constant, use the bitwise OR `|` operator.
+
+Example:
+
+```js
+const fs = require('fs');
+
+const {
+  O_RDWR,
+  O_CREAT,
+  O_EXCL
+} = fs.constants;
+
+fs.open('/path/to/my/file', O_RDWR | O_CREAT | O_EXCL, (err, fd) => {
+  // ...
+});
+```
+
 ### File Access Constants
 
 The following constants are meant for use with [`fs.access()`][].