Rollup merge of rust-lang#58606 - stjepang:put-future-into-spotlight,…

… r=alexcrichton

Docs: put Future trait into spotlight

If a function returns a type that implements `Future`, there should be a small "i" symbol next to it indicating the return type implements an important trait.

src/libcore/future/future.rs

@@ -24,6 +24,7 @@ use task::{Poll, Waker};
 ///
 /// When using a future, you generally won't call `poll` directly, but instead
 /// `await!` the value.
+#[doc(spotlight)]
 #[must_use = "futures do nothing unless polled"]
 pub trait Future {
     /// The type of value produced on completion.