alternatives: update doc to use the usual alternatives commands

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

modules/ROOT/pages/alternatives.adoc

@@ -35,3 +35,26 @@ 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]
+----
+# The legacy priority is 10, update nft to lower priority, so legacy wins
+pfx=/usr/sbin/iptables
+pfx6=/usr/sbin/ip6tables
+sudo alternatives --install ${pfx} iptables ${pfx}-nft 5 \
+  --slave ${pfx6} ip6tables ${pfx6}-nft \
+  --slave ${pfx}-restore iptables-restore ${pfx}-nft-restore \
+  --slave ${pfx}-save iptables-save ${pfx}-nft-save \
+  --slave ${pfx6}-restore ip6tables-restore ${pfx6}-nft-restore \
+  --slave ${pfx6}-save ip6tables-save ${pfx6}-nft-save
+sudo alternatives --auto iptables
+
+iptables --version
+----