Add a brief description to show up in http://static.rust-lang.org/doc/core/index.html

src/libcore/str/ascii.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+//! Operations on ASCII strings and characters.
+
 use to_str::{ToStr,ToStrConsume};
 use str;
 use cast;