Add guarantee about when a spawned task may be polled

tokio/src/task/spawn.rs

@@ -15,6 +15,12 @@ cfg_rt! {
     /// different thread to be executed. The specifics depend on the current
     /// [`Runtime`](crate::runtime::Runtime) configuration.
     ///
+    /// If the task is executed on the current thread it will not begin executing
+    /// until the current task yields to the executor. In other words, `spawn`
+    /// will not synchronously poll the task. As a result, calling `spawn`
+    /// whilst holding a lock does not pose a risk of deadlocking with the
+    /// spawned task.
+    ///
     /// There is no guarantee that a spawned task will execute to completion.
     /// When a runtime is shutdown, all outstanding tasks are dropped,
     /// regardless of the lifecycle of that task.