docs(networking.md): document the --rename option for interface-rename

[thomas-dkmt]

This PR updates the networking documentation to clarify that users can use the --rename option to modify network interface names on XCP-ng hosts.

Previously, only the --update option was documented, which caused confusion and sometimes prevented users from effectively renaming their NICs, as discussed on the XCP-ng forum.

[Fohdeesha]

Looks good to me, I agree we should keep the existing command first and foremost, that's what I have always used on systems without issue. I think the issue the user was getting in the forum, "eth0: ERROR while getting interface flags: No such device", after using update, is because the host needs to be rebooted after making these changes. It should not be used with either commands without a reboot afterwords, as a ton of stuff is already going to be bound to the old names on a running system

[stormi]

• A reminder about our commit message conventions. The limit is 70 chars per line. It's not arbitrary, it's a common convention for git and there are console tools that depend on that to be able to correctly display commit messages. https://docs.xcp-ng.org/project/development-process/commit-message-conventions/
• With the change made to the docs, it's more confusing to me. We give two nearly identical commands without explaining what the difference is. Also, the structure with "The common statement would be this... Or, in fact also that" doesn't work very well. And then we have "This example will set the mac-address for eth4 & eth8, switching them in the process.". Which example? We now give two of them, and they don't do the same thing according to the output of --help.

This PR should go further and properly explain what you can do with interface-rename.py, based on use cases.