Add doc example for OsString::reserve_exact.

Author: frewsxcv

src/libstd/ffi/os_str.rs

@@ -210,6 +210,16 @@ impl OsString {
     /// Note that the allocator may give the collection more space than it
     /// requests. Therefore capacity can not be relied upon to be precisely
     /// minimal. Prefer reserve if future insertions are expected.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::OsString;
+    ///
+    /// let mut s = OsString::new();
+    /// s.reserve_exact(10);
+    /// assert!(s.capacity() >= 10);
+    /// ```
     #[stable(feature = "osstring_simple_functions", since = "1.9.0")]
     pub fn reserve_exact(&mut self, additional: usize) {
         self.inner.reserve_exact(additional)