Add documentation example for str::Chars::as_str.

Author: frewsxcv

Diff for: src/libcore/str/mod.rs

@@ -459,6 +459,19 @@ impl<'a> Chars<'a> {
     ///
     /// This has the same lifetime as the original slice, and so the
     /// iterator can continue to be used while this exists.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut chars = "abc".chars();
+    ///
+    /// assert_eq!(chars.as_str(), "abc");
+    /// chars.next();
+    /// assert_eq!(chars.as_str(), "bc");
+    /// chars.next();
+    /// chars.next();
+    /// assert_eq!(chars.as_str(), "");
+    /// ```
     #[stable(feature = "iter_to_slice", since = "1.4.0")]
     #[inline]
     pub fn as_str(&self) -> &'a str {