Document using `tokio::time::sleep` (#111)

* Document using `tokio::time::sleep`

* Update reqwest-retry/src/middleware.rs

---------

Co-authored-by: Ethan Brierley <ethanboxx@gmail.com>

reqwest-retry/src/middleware.rs

@@ -12,7 +12,9 @@ use task_local_extensions::Extensions;
 /// and can be safely executed again.
 ///
 /// Currently, it allows setting a [RetryPolicy] algorithm for calculating the __wait_time__
-/// between each request retry.
+/// between each request retry. Sleeping on non-`wasm32` archs is performed using 
+/// [`tokio::time::sleep`], therefore it will respect pauses/auto-advance if run under a
+/// runtime that supports them.
 ///
 ///```rust
 ///     use reqwest_middleware::ClientBuilder;