Clarify effect of orphan rule changes on From/Into

Updated documentation for `std::convert` and `std::convert::From` to
reflect changes to orphan rule in Rust 1.41. It should no longer be
necessary to implement Into directly, unless targeting an older version.

Author: janikrabe

src/libcore/convert/mod.rs

@@ -18,8 +18,9 @@
 //! [`TryFrom<T>`][`TryFrom`] rather than [`Into<U>`][`Into`] or [`TryInto<U>`][`TryInto`],
 //! as [`From`] and [`TryFrom`] provide greater flexibility and offer
 //! equivalent [`Into`] or [`TryInto`] implementations for free, thanks to a
-//! blanket implementation in the standard library. Only implement [`Into`] or [`TryInto`]
-//! when a conversion to a type outside the current crate is required.
+//! blanket implementation in the standard library. When targeting a version prior to Rust 1.41, it
+//! may be necessary to implement [`Into`] or [`TryInto`] directly when converting to a type
+//! outside the current crate.
 //!
 //! # Generic Implementations
 //!
@@ -298,8 +299,10 @@ pub trait Into<T>: Sized {
 /// because implementing `From` automatically provides one with an implementation of [`Into`]
 /// thanks to the blanket implementation in the standard library.
 ///
-/// Only implement [`Into`] if a conversion to a type outside the current crate is required.
-/// `From` cannot do these type of conversions because of Rust's orphaning rules.
+/// Only implement [`Into`] when targeting a version prior to Rust 1.41 and converting to a type
+/// outside the current crate.
+/// `From` was not able to do these types of conversions in earlier versions because of Rust's
+/// orphaning rules.
 /// See [`Into`] for more details.
 ///
 /// Prefer using [`Into`] over using `From` when specifying trait bounds on a generic function.