Move historical reference into an editorial note

This is not a rule, but an editorial aside.

Author: ehuss

src/items/traits.md

@@ -93,8 +93,8 @@ A dyn-compatible trait can be the base trait of a [trait object]. A trait is
     * Explicitly non-dispatchable functions require:
         * Have a `where Self: Sized` bound (receiver type of `Self` (i.e. `self`) implies this).
 
-This concept was formerly known as *object safety*.
-The original set of rules was defined in [RFC 255] and has since been extended.
+> **Note**: This concept was formerly known as *object safety*.
+> The original set of rules were defined in [RFC 255] and have since been extended.
 
 ```rust
 # use std::rc::Rc;