fix generics section

Fixes rust-lang#17340

src/doc/guide-strings.md

@@ -96,12 +96,14 @@ need, and it can make your lifetimes more complex.
 
 ## Generic functions
 
-To write a function that's generic over types of strings, use [the `Str`
-trait](http://doc.rust-lang.org/std/str/trait.Str.html):
+To write a function that's generic over types of strings, you have two options:
+`&str`, or using [the `Str`
+trait](http://doc.rust-lang.org/std/str/trait.Str.html). Most of the time, you
+should prefer just taking `&str`:
 
 ```{rust}
-fn some_string_length<T: Str>(x: T) -> uint {
-        x.as_slice().len()
+fn some_string_length(x: &str) -> uint {
+        x.len()
 }
 
 fn main() {
@@ -111,14 +113,32 @@ fn main() {
 
     let s = "Hello, world".to_string();
 
-    println!("{}", some_string_length(s));
+    println!("{}", some_string_length(s.as_slice()));
 }
 ```
 
 Both of these lines will print `12`. 
 
-The only method that the `Str` trait has is `as_slice()`, which gives you
-access to a `&str` value from the underlying string.
+If you're using some kind of nested structure that has strings inside and you
+wish to be generic, you should use `Str`:
+
+```{rust}
+fn sum_of_string_length<T: Str>(xs: &[T]) -> uint {
+        xs.iter().fold(0, |acc, x| acc + x.as_slice().len())
+}
+
+fn main() {
+    let s = "Hello, world";
+
+    println!("{}", sum_of_string_length([s]));
+
+    let s = "Hello, world".to_string();
+
+    println!("{}", sum_of_string_length([s]));
+}
+```
+
+This is prefereable because it can save you allocating copies of what's inside.
 
 ## Comparisons