Improve consistency in executor documentation

futures/src/executor/async_std.rs

@@ -2,7 +2,7 @@ use crate::Executor;
 
 use futures::Future;
 
-/// A type representing the `async-std` runtime.
+/// An `async-std` runtime.
 #[derive(Debug)]
 pub struct AsyncStd;
 

futures/src/executor/thread_pool.rs

@@ -2,7 +2,7 @@ use crate::Executor;
 
 use futures::Future;
 
-/// A thread pool for futures.
+/// A thread pool runtime for futures.
 pub type ThreadPool = futures::executor::ThreadPool;
 
 impl Executor for futures::executor::ThreadPool {

futures/src/executor/tokio.rs

@@ -2,7 +2,7 @@ use crate::Executor;
 
 use futures::Future;
 
-/// The `tokio` runtime.
+/// A `tokio` runtime.
 pub type Tokio = tokio::runtime::Runtime;
 
 impl Executor for Tokio {

futures/src/executor/wasm_bindgen.rs

@@ -1,6 +1,6 @@
 use crate::Executor;
 
-/// A type representing a `wasm-bindgen-futures` runtime.
+/// A `wasm-bindgen-futures` runtime.
 #[derive(Debug)]
 pub struct WasmBindgen;