docs: annotate io mod with doc_cfg (#1808)

Annotates types in `tokio::io` module with their required feature flag.
This annotation is included in generated documentation.

Notes:

* The annotation must be on the type or function itself. Annotating just
  the re-export is not sufficient.

* The annotation must be **inside** the `pin_project!` macro or it is
  lost.

tokio/src/process/mod.rs

@@ -101,10 +101,14 @@
 //! While similar to the standard library, this crate's `Child` type differs
 //! importantly in the behavior of `drop`. In the standard library, a child
 //! process will continue running after the instance of [`std::process::Child`]
-//! is dropped. In this crate, however, because [`tokio::process::Child`][Child] is a
+//! is dropped. In this crate, however, because [`tokio::process::Child`] is a
 //! future of the child's `ExitStatus`, a child process is terminated if
 //! `tokio::process::Child` is dropped. The behavior of the standard library can
-//! be regained with the [`Child::forget`](crate::process::Child::forget) method.
+//! be regained with the [`Child::forget`](crate::process::Child::forget)
+//! method.
+//!
+//! [`Command`]: crate::process::Command
+//! [`tokio::process::Child`]: crate::process::Child
 
 #[path = "unix/mod.rs"]
 #[cfg(unix)]