doc: add examples for to_uppercase and to_lowercase

Author: tshepang

src/librustc_unicode/char.rs

@@ -467,6 +467,12 @@ impl char {
     /// Returns an iterator which yields the characters corresponding to the
     /// lowercase equivalent of the character. If no conversion is possible then
     /// an iterator with just the input character is returned.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// assert_eq!(Some('c'), 'C'.to_lowercase().next());
+    /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn to_lowercase(self) -> ToLowercase {
@@ -515,6 +521,12 @@ impl char {
     /// [`SpecialCasing.txt`]: ftp://ftp.unicode.org/Public/UNIDATA/SpecialCasing.txt
     ///
     /// [2]: http://www.unicode.org/versions/Unicode7.0.0/ch03.pdf#G33992
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// assert_eq!(Some('C'), 'c'.to_uppercase().next());
+    /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn to_uppercase(self) -> ToUppercase {