doc: add example code for process.getgroups()

PR-URL: nodejs#35625
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

doc/api/process.md

@@ -1342,6 +1342,12 @@ The `process.getgroups()` method returns an array with the supplementary group
 IDs. POSIX leaves it unspecified if the effective group ID is included but
 Node.js ensures it always is.
 
+```js
+if (process.getgroups) {
+  console.log(process.getgroups()); // [ 16, 21, 297 ]
+}
+```
+
 This function is only available on POSIX platforms (i.e. not Windows or
 Android).