Incompatible feature flags and documentation

[KillTheMule]

The feature flags use_tokio and use_async-std are incompatible (and will be incompatible with the planned use_glib as well). The reason is overlapping impl's of the Spawner trait.

On the one hand, it's not a real problem, there will probably no one using this create with more than one runtime.

On the other hand, that means we can enable only one of those flags for docs.rs, so that documentation remains incomplete.

If anyone has an idea around that, I'd be very happy about improving this situation.