Implemented #[doc(cfg(...))].

This attribute has two effects:

1. Items with this attribute and their children will have the "This is
   supported on **** only" message attached in the documentation.

2. The items' doc tests will be skipped if the configuration does not
   match.

src/librustdoc/lib.rs

@@ -26,6 +26,7 @@
 #![feature(test)]
 #![feature(unicode)]
 #![feature(vec_remove_item)]
+#![feature(ascii_ctype)]
 
 extern crate arena;
 extern crate getopts;