Update docs

- Improve IKEv2 docs. The strongSwan Android VPN client requires
  an "IP address" in the VPN server certificate's subjectAltName field
  in addition to "DNS name", when connecting using the server's IP.
  The certutil commands have been updated to add this field.
- Other improvements to docs

docs/manage-users.md

@@ -35,4 +35,9 @@ Passwords in this file are salted and hashed. This step can be done using e.g. t
 openssl passwd -1 "<VPN Password 1>"
 ```
 
-When finished making changes, reboot your server.
+When finished, restart services:
+
+```bash
+service ipsec restart
+service xl2tpd restart
+```