Improve documentation for built-in macros

Author: petrochenkov

src/libcore/lib.rs

@@ -75,6 +75,7 @@
 #![feature(const_fn)]
 #![feature(const_fn_union)]
 #![feature(custom_inner_attributes)]
+#![feature(decl_macro)]
 #![feature(doc_cfg)]
 #![feature(doc_spotlight)]
 #![feature(extern_types)]