Add doc example for OsString::shrink_to_fit.

src/libstd/ffi/os_str.rs

@@ -226,6 +226,22 @@ pub fn reserve_exact(&mut self, additional: usize) {
     }
 
     /// Shrinks the capacity of the `OsString` to match its length.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(osstring_shrink_to_fit)]
+    ///
+    /// use std::ffi::OsString;
+    ///
+    /// let mut s = OsString::from("foo");
+    ///
+    /// s.reserve(100);
+    /// assert!(s.capacity() >= 100);
+    ///
+    /// s.shrink_to_fit();
+    /// assert_eq!(3, s.capacity());
+    /// ```
     #[unstable(feature = "osstring_shrink_to_fit", issue = "40421")]
     pub fn shrink_to_fit(&mut self) {
         self.inner.shrink_to_fit()