doc(str): show example of chars().count() under len()

the docs are great at explaining that .len() isn't like in other
languages but stops short of explaining how to get the character length.

r? @steveklabnik

src/liballoc/string.rs

@@ -1402,7 +1402,9 @@ impl String {
         &mut self.vec
     }
 
-    /// Returns the length of this `String`, in bytes.
+    /// Returns the length of this `String`, in bytes, not [`char`]s or
+    /// graphemes. In other words, it may not be what a human considers the
+    /// length of the string.
     ///
     /// # Examples
     ///
@@ -1410,8 +1412,11 @@ impl String {
     ///
     /// ```
     /// let a = String::from("foo");
-    ///
     /// assert_eq!(a.len(), 3);
+    ///
+    /// let fancy_f = String::from("ƒoo");
+    /// assert_eq!(fancy_f.len(), 4);
+    /// assert_eq!(fancy_f.chars().count(), 3);
     /// ```
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]

src/libcore/str/mod.rs

@@ -2085,8 +2085,8 @@ impl str {
     /// let len = "foo".len();
     /// assert_eq!(3, len);
     ///
-    /// let len = "ƒoo".len(); // fancy f!
-    /// assert_eq!(4, len);
+    /// assert_eq!("ƒoo".len(), 4); // fancy f!
+    /// assert_eq!("ƒoo".chars().count(), 3);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]