Add a short explanation for "pipeline" in tutorial "Channel" chapter (#823)

Author: Emil-Stampfly-He

content/tokio/tutorial/channels.md

@@ -66,7 +66,9 @@ means that exclusive access is required to call it. We could open a connection
 per task, but that is not ideal. We cannot use `std::sync::Mutex` as `.await`
 would need to be called with the lock held. We could use `tokio::sync::Mutex`,
 but that would only allow a single in-flight request. If the client implements
-[pipelining], an async mutex results in underutilizing the connection.
+[pipelining] (in short, sending many commands without waiting for each prior
+command's response), an async mutex results in underutilizing
+the connection.
 
 [pipelining]: https://redis.io/topics/pipelining