improve docstring

Author: carstenbauer

src/internals.jl

@@ -78,6 +78,8 @@ macro spawn(args...)
 end
 
 """
+    @spawnat thrdid expr
+
 Similar to `StableTasks.@spawn` but creates a **sticky** `Task` and schedules it to run on the thread with the given id (`thrdid`).
 The task is guaranteed to stay on this thread (it won't migrate to another thread).
 """