auto merge of #7792 : ozten/rust/master, r=huonw

Minor tweak, but I was confused when first digging into json.rs docs.

I think it's clearer to say the module provides parsing and serialization.

src/libextra/json.rs

@@ -14,7 +14,7 @@
 #[forbid(non_camel_case_types)];
 #[allow(missing_doc)];
 
-//! json serialization
+//! json parsing and serialization
 
 use std::iterator;
 use std::float;