If you need an explicit written sentence on the wiki to tell you that OSM is a community project then you probably need to work on your understanding how OSM works. You aren't mapping alone in a green field and need to work in collaboration with the (local) community. 

Your definition of what a 'tunnel' is and isn't has no bearing on what the `tunnel=*` tag means and is used for in OSM. You need to work with established OSM definitions and not try to single-handedly make OSM fit your world view.

In OSM, the `tunnel=*` tag is used for a wide variety of things that are 'artificial'/'man made' passages for vehicles, people, cables/infrastructure, water, etc... to pass through something else like a mountain, soil or even buildings.

Whether it is underground or not is independent of the fact that it is a 'tunnel'.
The `layer=*` tag also only specifies the relative height of overlapping features and similarly has no influence on whether something is a tunnel or not. It also does not specify that something is located underground.

Of course most tunnels (that aren't `tunnel=building_passage/avalanche_protector`) are indeed underground and that should probably be assumed as the default so that we don't need to add `location=underground` to all of them. For the exceptions that aren't, `location=overground` should be added instead.
From that perspective, adding a `tunnel=*` tag _does_ strongly imply that it is underground, even if it does not explicitly specify it. Tagging `location=underground` in addition would also be correct (even if possibly slightly redundant).