Document DynDNS behaviour with no "myip" parameter

Currently, the documentation for DynDNS makes it look like the myip query parameter is mandatory:

Depending on the client you are using, an update URL may be required, in addition to the DynHost username and subdomain concerned. If this is the case, use the URL below, taking care to replace the generic information:

https://dns.eu.ovhapis.com/nic/update?system=dyndns&hostname=$HOSTNAME&myip=$IP

However, it is possible to query without the myip parameter, such as:

curl https://dns.eu.ovhapis.com/nic/update?system=dyndns&hostname=$HOSTNAME

In which case OVH simply uses your current public IP as the new IP.

This is not documented anywhere, and probably should since it simplifies the update, removing the need to perform a public address discovery using an extra GET to pass in myip.

Start with pages/web_cloud/domains/dns_dynhost/guide.en-gb.md, linked in the issue. Review the existing DynDNS update URL example and document the behavior when myip is omitted, including the provided curl request. Done means the guide clearly explains this simpler update path without implying that myip is mandatory.