Update duration docs for oldtime feature

The main crate docs made some claims about the Duration type that became
incorrect with the new oldtime feature (chronotope#478).

Per @jhpratt.

src/lib.rs

@@ -64,15 +64,22 @@
 //!
 //! ### Duration
 //!
-//! Chrono currently uses
-//! the [`time::Duration`](https://docs.rs/time/0.1.40/time/struct.Duration.html) type
-//! from the `time` crate to represent the magnitude of a time span.
-//! Since this has the same name as the newer, standard type for duration,
-//! the reference will refer this type as `OldDuration`.
+//! Chrono currently uses its own [`Duration`] type to represent the magnitude
+//! of a time span. Since this has the same name as the newer, standard type for
+//! duration, the reference will refer this type as `OldDuration`.
+//!
 //! Note that this is an "accurate" duration represented as seconds and
 //! nanoseconds and does not represent "nominal" components such as days or
 //! months.
 //!
+//! When the `oldtime` feature is enabled, [`Duration`] is an alias for the
+//! [`time::Duration`](https://docs.rs/time/0.1.40/time/struct.Duration.html)
+//! type from v0.1 of the time crate. time v0.1 is deprecated, so new code
+//! should disable the `oldtime` feature and use the `chrono::Duration` type
+//! instead. The `oldtime` feature is enabled by default for backwards
+//! compatibility, but future versions of Chrono are likely to remove the
+//! feature entirely.
+//!
 //! Chrono does not yet natively support
 //! the standard [`Duration`](https://doc.rust-lang.org/std/time/struct.Duration.html) type,
 //! but it will be supported in the future.