Future proofed exception paragraph in runtime compatibility section

Co-authored-by: Alice Ryhl <aliceryhl@google.com>

tokio/src/sync/mod.rs

@@ -442,10 +442,9 @@
 //! [cooperative scheduling](crate::task#cooperative-scheduling) to avoid
 //! starvation. This feature does not apply when used from non-Tokio runtimes.
 //!
-//! The only exception to runtime compatibility is the
-//! [`mpsc::Sender::send_timeout`] method, which can't be called from non-Tokio
-//! runtimes. However, [`Senders`](mpsc::Sender) can still be moved between
-//! different instances of the Tokio runtime.
+//! As an exception, methods ending in `_timeout` are not runtime agnostic
+//! because they require access to the Tokio timer. See the documentation of
+//! each `*_timeout` method for more information on its use.
 
 cfg_sync! {
     /// Named future types.