Overall, this looks pretty good to start with and I think it's potentially very useful. A way to interrogate this text would be to go back through the Help forums to pick out the threads where there was a communication barrier and see if the explanation covers that use case in a way where linking to the section of the new doc would help explain what's going on. For example, the second response in this thread (Could not issue a Let's Encrypt SSL/TLS certificate - #2 by JuergenAuer) isn't covered under any of the situations and a solution would be to further define what a domain is under the first paragraph of Situation 1. Something simple like "www.example.com" is a domain and "your.example.com" is a subdomain.
How far back would you like for me to go?