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

diff --cc src/libstd_unicode/char.rs
Simple merge