Add doc aliases sleep and timeout to Timer (#242)

Two examples are shown in `Timer`'s docs:
One example for basic _sleeping_ and one example for having a _timeout_ on a future.

This adds corresponding doc aliases to `Timer`, so that this struct is easier to find with those use cases in mind.

Author: janriemer

src/lib.rs

@@ -138,6 +138,8 @@ pub use reactor::{Readable, ReadableOwned, Writable, WritableOwned};
 ///     .await?;
 /// # std::io::Result::Ok(()) });
 /// ```
+#[doc(alias = "sleep")]
+#[doc(alias = "timeout")]
 #[derive(Debug)]
 pub struct Timer {
     /// This timer's ID and last waker that polled it.