Chapter 12.5 should mention that search with to_lowercase is not correct with Unicode

[malaire]

The search_case_insensitive example in chapter 12.5 should mention that this solution of using to_lowercase for case insensitive matching is not correct for Unicode and is only used to make the example simple.

For example this test fails when using to_lowercase:

    #[test]
    fn case_insensitive_unicode() {
        let query = "weiß";
        let contents = "WEISS";

        assert_eq!(
            vec!["WEISS"],
            search_case_insensitive(query, contents)
        );
    }

ps. Correct solution would be to use case folding, but I don't know if that is available in Rust.