task: clarify when to use spawn_blocking vs dedicated threads

tokio/src/task/blocking.rs

@@ -103,6 +103,21 @@ cfg_rt! {
     /// their own. If you want to spawn an ordinary thread, you should use
     /// [`thread::spawn`] instead.
     ///
+    /// # When to use `spawn_blocking` vs dedicated threads
+    ///
+    /// `spawn_blocking` is intended for *bounded* blocking work that eventually finishes.
+    /// Each call occupies a thread from the runtime's blocking thread pool for the duration
+    /// of the task. Long-lived tasks therefore reduce the pool's effective capacity, which
+    /// can delay other blocking operations once the pool is saturated and work is queued.
+    ///
+    /// For workloads that run indefinitely or for extended periods (for example,
+    /// background workers or persistent processing loops), prefer a dedicated thread created with
+    /// [`thread::spawn`].
+    ///
+    /// As a rule of thumb:
+    /// - Use `spawn_blocking` for short-lived blocking operations
+    /// - Use dedicated threads for long-lived or persistent blocking workloads
+    ///
     /// Be aware that tasks spawned using `spawn_blocking` cannot be aborted
     /// because they are not async. If you call [`abort`] on a `spawn_blocking`
     /// task, then this *will not have any effect*, and the task will continue