Tiny improvement to docs for core::convert.

This is not really significant, accept or reject as you wish.  I
just want to make sure I understand how the PR process works and
I'm doing it right before doing a bigger one for rust-lang#33417.

Author: icefoxen

src/libcore/convert.rs

@@ -17,7 +17,10 @@
 //! [`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.
+//! blanket implementation in the standard library.  However, there are some cases
+//! where this is not possible, such as creating conversions into a type defined
+//! outside your library, so implementing [`Into`] instead of [`From`] is
+//! sometimes necessary.
 //!
 //! # Generic Implementations
 //!