[CHORE] Explain block_on function in common-runtime (#3442)

Addresses: #3435

---------

Co-authored-by: Colin Ho <colinho@Colins-MacBook-Pro.local>

src/common/runtime/src/lib.rs

@@ -100,7 +100,12 @@ impl Runtime {
 
     /// Spawns a task on the runtime and blocks the current thread until the task is completed.
     /// Similar to tokio's Runtime::block_on but requires static lifetime + Send
-    /// You should use this when you are spawning IO tasks from an Expression Evaluator or in the Executor
+    /// You should use this when you need to run an async task in a synchronous function, but you are already in a tokio runtime.
+    ///
+    /// For example, URL download is an async function, but it is called from a synchronous function in a tokio runtime,
+    /// i.e. calling the Expression Evaluator from the Native Executor.
+    ///
+    /// In the future, we should refactor the code to be fully async, but for now, this is a workaround.
     pub fn block_on<F>(&self, future: F) -> DaftResult<F::Output>
     where
         F: Future + Send + 'static,