Rollup merge of #40499 - ericfindlay:master, r=steveklabnik

Corrected very minor documentation detail about Unicode and Japanese

Japanese half-width and full-width romaji characters do have upper and lowercase according Unicode (but other Japanese characters do not). For example,
` assert_eq!('\u{FF21}'.to_lowercase().collect::<String>(),"\u{FF41}");`

r? @steveklabnik

src/libstd_unicode/char.rs

@@ -829,7 +829,8 @@ impl char {
     /// // Sometimes the result is more than one character:
     /// assert_eq!('İ'.to_lowercase().to_string(), "i\u{307}");
     ///
-    /// // Japanese scripts do not have case, and so:
+    /// // Characters that do not have both uppercase and lowercase
+    /// // convert into themselves.
     /// assert_eq!('山'.to_lowercase().to_string(), "山");
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
@@ -889,7 +890,8 @@ impl char {
     /// // Sometimes the result is more than one character:
     /// assert_eq!('ß'.to_uppercase().to_string(), "SS");
     ///
-    /// // Japanese does not have case, and so:
+    /// // Characters that do not have both uppercase and lowercase
+    /// // convert into themselves.
     /// assert_eq!('山'.to_uppercase().to_string(), "山");
     /// ```
     ///