You’re showcasing a principal problem in doc writing.
We all, and advanced users even more so, interpret and infer things at all times and there’s little a doc writer can reasonably anticipate about it when he’s adressing a wide or even unknown audience.
People’s inferences are fairly often incorrect such as yours here. I myself would not have interpreted it so but it’s common knowledge that never “everyone will read it the way it was intended”.
Not saying we cannot improve wording, it’s more important however that I would expect that a reader, in case of doubt and at least after he got confused or he had tried and it didn’t work as expected, should question himself if he or she misunderstood the meaning rather than blame the docs they made him misunderstand or even question the software.
Sure, that’s a spectrum and the amount of tolerance they bring varies among people and is dependent on their predisposition, knowledge and situation e.g. how much they are in a hurry, but clarifying bad inferences (that happened for real) already make up for a substantial part of the current docs which in turn leads to those sometimes feeling to be lectured like @JimT.
It’s part of the problem that we as doc writers, advanced users or devs, have a different bias ourselves than ‘normal’ users do, and we’re a minority so we should be inferring less ourselves and think more about how most would read it.
That being said, any improvement is welcome, looking forward to your proposal. Be aware however that @rpwong volunteered for a rewrite, so maybe better post it here than setting it up as a PR.
Hmm, reading the document, service workers (which MainUI uses) are (or at least will be) only HTTPS, as well as location access, so there are more arguments to use HTTPS.
I guess we should improve the openHABian docs to explain why to use HTTPS and how to setup your system to trust openHAB‘s self-signed certificate.
Please don’t mix openHAB and openHABian. It’s tiring enough that users do 
And no we cannot, and I don’t want to explain all the basics including reasoning. That’s known to lead to even more discussion and people feeling to be lectured.
Doc writing has many aspects. It’s a tough and underestimated challenge.
Sure 
However I think this should at least be referenced from the openHABian docs, since I guess most users that manually install openHAB are likely to already know about HTTPS and self-signed certificates.
FYI I have created a feature request to have a REST endpoint where the server certificate can be retrieved from, so we can add a „Download server certificate“ button in the UI.
See REST endpoint to get Jetty‘s TLS certificate · Issue #4130 · openhab/openhab-core · GitHub
The challenge is http://openhabian:81 is not guaranteed to work on everyone’s LAN. In those cases the IP address instead of the hostname is the only way to reach the machine.