The existing Luci Wireless UI has a warning under the channel selection dropdown: "Some channels may be restricted to Indoor Only use by your Regulatory Domain. Make sure to follow this advice if a channel is reported as such." Considering that the ramifications of making a poor choice of channel can be quite expensive due to radar interference, it would be helpful to have a more thorough explanation, or ideally a popup that warns the user when attempting to force such a channel.
The indoor/outdoor interpretation varies from place to place. Local techs doing professional outdoor installations know their trade to set option country3 'O' for directional access point with antennas set up outside fixed building.
Yeah a link to a Wikipedia page about wifi channels would be fine, assuming it has a section on spectral restrictions, which as you indicated may vary from country to country.
But even with this notation, how many people are going to figure out that their wifi just persistently disabled itself because of a transient conflict with some other spectrum user? It's just not intuitive to think that sort of thing should spontaneously occur on healthy hardware, let alone persist across reboot, so it starts to look like a hardware failure.
In light of this, I might add: "Disregarding channel constraints may lead to the wifi being disabled in a manner which persists across reboots." Or in all-caps because this is so unintuitive in light of the straightforward channel selection UI. (Like you can't press "7" on your calculator at the same time as anyone else in your office, or it will stop working until you reinstall the firmware. There needs to be a big red warning sticker on the side.)