task: clarify when to use spawn_blocking vs dedicated threads (#7923)

tokio/src/task/blocking.rs

@@ -119,6 +119,21 @@ cfg_rt! {
     /// returns. It is possible for a blocking task to be cancelled if it has
     /// not yet started running, but this is not guaranteed.
     ///
+    /// # 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
+    ///
     /// Note that if you are using the single threaded runtime, this function will
     /// still spawn additional threads for blocking operations. The current-thread
     /// scheduler's single thread is only used for asynchronous code.