Update step-down.mdx (#19329)

Co-authored-by: Violet Hynes <violet.hynes@hashicorp.com>
Co-authored-by: Sarah Chavis <62406755+schavis@users.noreply.github.com>

website/content/docs/commands/operator/step-down.mdx

@@ -31,3 +31,14 @@ Success! Stepped down: http://127.0.0.1:8200
 
 There are no flags beyond the [standard set of flags](/vault/docs/commands)
 included on all commands.
+
+## Policy requirements
+
+You must belong to a policy with `update` and `sudo` permissions to use the
+`vault operator step-down` command:
+ 
+```hcl
+path "sys/step-down" {
+  capabilities = ["update", "sudo"]
+}
+```