I understand that generally the documentation is fixed. In this case, however, the parameter is relatively new and only used as a boolean on this wiki. This is preferred as it aligns with the TemplateData spec, and means that it is optimally supported by VisualEditor and other systems in the future.
It was documented correctly. It is true that it unofficially could also be set with inline=whatever
but was not documented and not supported. If that workedk, it would have been a mistake. I searched using insource:
on this wiki, just in case, and found no results. If we do find some, it's easy to fix.
I'm well-aware of the meaning of the boolean type, given as I wrote the TemplateData spec that defines it. The third "unknown" state does not mean that it has to behave differently from 1 or 0. It simply means that the editing user has not defined it. Which means the choice is not significant and either 1 or 0 is fine, for the template to decide.
The default can be simple like here, where the default is just 0. Or, the default can be complicated, based on the namespace or other parameters etc.
I'd like to understand better how you interpret the meaning of the third "special" value. What did it mean to you?