Document async_fn_traits

Author: oli-obk

src/doc/unstable-book/src/library-features/async-fn-traits.md

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+# `async_fn_traits`
+
+See Also: [`fn_traits`](../library-features/fn-traits.md)
+
+----
+
+The `async_fn_traits` feature allows for implementation of the [`AsyncFn*`] traits
+for creating custom closure-like types that return futures.
+
+[`AsyncFn*`]: ../../std/ops/trait.AsyncFn.html
+
+The main difference to the `Fn*` family of traits is that `AsyncFn` can return a future
+that borrows from itself (`FnOnce::Output` has no lifetime parameters, while `AsyncFn::CallFuture` does).