alternatives: update doc to use the usual alternatives commands

See coreos/fedora-coreos-tracker#677 (comment)

modules/ROOT/pages/alternatives.adoc

@@ -35,3 +35,24 @@ storage:
       overwrite: true
       hard: false
 ----
+
+== Manual setup
+
+Starting with Fedora CoreOS based on Fedora 41, you can use `alternatives` commands to configure the default command manually.
+
+For example, to use the legacy-based variants of the `iptables` commands:
+
+.Example using alternatives to configure default command manually
+[source,bash]
+----
+# Check the link info
+alternatives --display iptables
+iptables --version
+
+# Config iptables to point to legacy
+sudo alternatives --set iptables /usr/sbin/iptables-legacy
+alternatives --display iptables
+
+# Verify iptables version is legacy
+iptables --version
+----