link to Future::poll from the Poll docs

The most important thing about Poll is that Future::poll returns it, but
previously the docs didn't emphasize this.

Author: oconnor663

library/core/src/task/poll.rs

@@ -5,6 +5,10 @@ use crate::ops::{self, ControlFlow};
 
 /// Indicates whether a value is available or if the current task has been
 /// scheduled to receive a wakeup instead.
+///
+/// This is the return type of <code>[Future::poll]</code>.
+///
+/// [Future::poll]: ../future/trait.Future.html "Future"
 #[must_use = "this `Poll` may be a `Pending` variant, which should be handled"]
 #[derive(Copy, Clone, Debug, Eq, PartialEq, Ord, PartialOrd, Hash)]
 #[lang = "Poll"]