fix safety docs

tokio/src/runtime/scheduler/current_thread/mod.rs

@@ -468,6 +468,9 @@ impl Handle {
     /// Spawn a task which isn't safe to send across thread boundaries onto the runtime.
     ///
     /// # Safety
+    /// This should only be used when this is a LocalRuntime or in another case where the runtime
+    /// provably cannot be driven from or moved to different threads from the one on which the task
+    /// is spawned.
     pub(crate) unsafe fn spawn_local<F>(
         me: &Arc<Self>,
         future: F,