Adds some additional helper text clarifying the need for more than one domain name to exist before cross domain linking can be used.
Adds a new i18n key
I explored having this additional text only show up if a user has only 1 domain name set up, but this information was not already available in the
Site data cache (that I could find), and I did not think it warranted a database request. Let me know if you disagree and would like me to add the behavior I described (only show text if user only has 1 domain set up).