Basically similar to internal interfaces (see question C-5-8 (internal interfaces)).
The major distinction might be the fact that external people (beyond the scope and influence of your systems’ stakeholders) can be involved, both as consumers and as providers of interfaces (or data or events at these interfaces).
As these external people will likely not have access to your internal arc42 documentation, you might be required to provide distinct documents for your external interfaces.
Changing external interfaces will usually be much harder than modifications at internal interfaces - simply because external stakeholders will be involved (whom you cannot control or influence). Therefore external interfaces contain greater risks than internal interfaces, and they need more attention, care and management. As software architect, these pesky things belong to your responsibility.