docs: Update num_alive_tasks docs

Author: joe-amzn

tokio/src/runtime/metrics/runtime.rs

@@ -53,6 +53,10 @@ impl RuntimeMetrics {
     ///
     /// This counter increases when a task is spawned and decreases when a
     /// task exits.
+    /// 
+    /// Note: When using the multi-threaded runtime this number may not
+    /// not have strong consistency i.e. no tasks may be running but the metric
+    /// reports otherwise.
     ///
     /// # Examples
     ///