Improve quality of String#to_slice() deprecation message

Author: steveklabnik

src/libcore/str/mod.rs

@@ -1150,7 +1150,7 @@ mod traits {
 
 /// Any string that can be represented as a slice
 #[unstable = "Instead of taking this bound generically, this trait will be \
-              replaced with one of slicing syntax, deref coercions, or \
+              replaced with one of slicing syntax (&foo[]), deref coercions, or \
               a more generic conversion trait"]
 pub trait Str {
     /// Work with `self` as a slice.