Better docs for TxPool::import_notification_stream (#4157)

* better docs for import_notification_stream

* Apply suggestions from code review

Co-Authored-By: Tomasz Drwięga <tomusdrw@users.noreply.github.com>

substrate/client/transaction-pool/graph/src/pool.rs

@@ -279,7 +279,10 @@ impl<B: ChainApi> Pool<B> {
 			)))
 	}
 
-	/// Return an event stream of transactions imported to the pool.
+	/// Return an event stream of notifications for when transactions are imported to the pool.
+	/// 
+	/// Consumers of this stream should use the `ready` method to actually get the
+	/// pending transactions in the right order.
 	pub fn import_notification_stream(&self) -> EventStream {
 		self.validated_pool.import_notification_stream()
 	}