Add a note into the docstring of sensor types that special characters should be avoided.

[EdwinB12]

Special characters are causing problems. We don't have time to solve the exact problem, but a note in the docstring would be helpful.

[mhauru]

I'm still unclear on whether this is an inherent issue with Unicode characters in the fields like sensor_unique_id, like maybe PostgreSQL mangles them or something, or if the problem is just that some Unicode characters look very much like each other, which does cause trouble for users, but I think isn't anything we can really solve.