Add doctests for directionality

Thanks @nodakai

Author: steveklabnik

src/libcollections/str.rs

@@ -1531,6 +1531,16 @@ impl str {
     ///
     /// assert_eq!("Hello\tworld\t", s.trim_left());
     /// ```
+    ///
+    /// Directionality:
+    ///
+    /// ```
+    /// let s = "  English";
+    /// assert!(Some('E') == s.trim_left().chars().next());
+    ///
+    /// let s = "  עברית";
+    /// assert!(Some('ע') == s.trim_left().chars().next());
+    /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn trim_left(&self) -> &str {
         UnicodeStr::trim_left(self)
@@ -1557,6 +1567,16 @@ impl str {
     ///
     /// assert_eq!(" Hello\tworld", s.trim_right());
     /// ```
+    ///
+    /// Directionality:
+    ///
+    /// ```
+    /// let s = "English  ";
+    /// assert!(Some('E') == s.trim_right().chars().next());
+    ///
+    /// let s = "עברית  ";
+    /// assert!(Some('ת') == s.trim_right().chars().next());
+    /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn trim_right(&self) -> &str {
         UnicodeStr::trim_right(self)