Minor fixes in async (#771)

src/async/channels.md

@@ -1,6 +1,6 @@
 # Async Channels
 
-Several crates have support for `async`/`await`. For instance `tokio` channels:
+Several crates have support for asynchronous channels. For instance `tokio`:
 
 ```rust,editable,compile_fail
 use tokio::sync::mpsc::{self, Receiver};

src/async/control-flow/select.md

@@ -5,9 +5,10 @@ that future's result. In JavaScript, this is similar to `Promise.race`. In
 Python, it compares to `asyncio.wait(task_set,
 return_when=asyncio.FIRST_COMPLETED)`.
 
-This is usually a macro, similar to match, with each arm of the form `pattern =
-future => statement`. When the future is ready, the statement is executed with the
-variable bound to the future's result.
+Similar to a match statement, the body of `select!` has a number of arms, each
+of the form `pattern = future => statement`. When the `future` is ready, the
+`statement` is executed with the variables in `pattern` bound to the `future`'s
+result.
 
 ```rust,editable,compile_fail
 use tokio::sync::mpsc::{self, Receiver};

src/async/tasks.md

@@ -1,7 +1,6 @@
 # Tasks
 
-Runtimes have the concept of a "task", similar to a thread but much
-less resource-intensive.
+Rust has a task system, which is a form of lightweight threading.
 
 A task has a single top-level future which the executor polls to make progress.
 That future may have one or more nested futures that its `poll` method polls,