api: document goroutine safeness

[stapelberg]

From what I can see, both Client and QueryAPI are safe for use from multiple goroutines as they don’t modify any state. Let’s document this so that others don’t need to review the source as well :).

[beorn7]

Totally for documenting goroutine safety. But to make the call here, we need @fabxc .

[fabxc]

Thanks 👍