docs: Improve char::to_{lower,upper}case examples

Collect the results to a String to make it clear that it will not always
return only one char and add examples showing that.

Author: ollie27

src/librustc_unicode/char.rs

@@ -668,10 +668,13 @@ impl char {
     /// Basic usage:
     ///
     /// ```
-    /// assert_eq!('C'.to_lowercase().next(), Some('c'));
+    /// assert_eq!('C'.to_lowercase().collect::<String>(), "c");
+    ///
+    /// // Sometimes the result is more than one character:
+    /// assert_eq!('İ'.to_lowercase().collect::<String>(), "i\u{307}");
     ///
     /// // Japanese scripts do not have case, and so:
-    /// assert_eq!('山'.to_lowercase().next(), Some('山'));
+    /// assert_eq!('山'.to_lowercase().collect::<String>(), "山");
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
@@ -702,10 +705,13 @@ impl char {
     /// Basic usage:
     ///
     /// ```
-    /// assert_eq!('c'.to_uppercase().next(), Some('C'));
+    /// assert_eq!('c'.to_uppercase().collect::<String>(), "C");
+    ///
+    /// // Sometimes the result is more than one character:
+    /// assert_eq!('ß'.to_uppercase().collect::<String>(), "SS");
     ///
     /// // Japanese does not have case, and so:
-    /// assert_eq!('山'.to_uppercase().next(), Some('山'));
+    /// assert_eq!('山'.to_uppercase().collect::<String>(), "山");
     /// ```
     ///
     /// In Turkish, the equivalent of 'i' in Latin has five forms instead of two:
@@ -716,17 +722,17 @@ impl char {
     /// Note that the lowercase dotted 'i' is the same as the Latin. Therefore:
     ///
     /// ```
-    /// let upper_i = 'i'.to_uppercase().next();
+    /// let upper_i: String = 'i'.to_uppercase().collect();
     /// ```
     ///
     /// The value of `upper_i` here relies on the language of the text: if we're
-    /// in `en-US`, it should be `Some('I')`, but if we're in `tr_TR`, it should
-    /// be `Some('İ')`. `to_uppercase()` does not take this into account, and so:
+    /// in `en-US`, it should be `"I"`, but if we're in `tr_TR`, it should
+    /// be `"İ"`. `to_uppercase()` does not take this into account, and so:
     ///
     /// ```
-    /// let upper_i = 'i'.to_uppercase().next();
+    /// let upper_i: String = 'i'.to_uppercase().collect();
     ///
-    /// assert_eq!(Some('I'), upper_i);
+    /// assert_eq!(upper_i, "I");
     /// ```
     ///
     /// holds across languages.