Partially audit the trait objects section.

Mention object-safety for traits, link to RFC 255.

Author: Nick Hamann

src/doc/reference.md

@@ -3387,11 +3387,12 @@ more of the closure traits:
 ### Trait objects
 
 Every trait item (see [traits](#traits)) defines a type with the same name as
-the trait. This type is called the _trait object_ of the trait. Trait objects
-permit "late binding" of methods, dispatched using _virtual method tables_
-("vtables"). Whereas most calls to trait methods are "early bound" (statically
-resolved) to specific implementations at compile time, a call to a method on an
-trait objects is only resolved to a vtable entry at compile time. The actual
+the trait. Traits that are object-safe (see [RFC 255][rfc255]) can be
+instantiated as _trait objects_, which permit "late binding" of methods,
+dispatched using _virtual method tables_ ("vtables"). Whereas most calls to
+trait methods are "early bound" (statically resolved) to specific
+implementations at compile time, a call to a method on an trait objects is
+only resolved to a vtable entry at compile time. The actual
 implementation for each vtable entry can vary on an object-by-object basis.
 
 Given a pointer-typed expression `E` of type `&T` or `Box<T>`, where `T`
@@ -3679,3 +3680,4 @@ that have since been removed):
 
 [ffi]: book/ffi.html
 [plugin]: book/compiler-plugins.html
+[rfc255]: https://github.com/rust-lang/rfcs/blob/master/text/0255-object-safety.md