Disable the JSON doctests because they don't pass the new coherence

rules and cannot be updated until the libraries are synced, nor can
they opt in to the old semantics.

Author: nikomatsakis

src/libserialize/json.rs

@@ -76,7 +76,8 @@
 //! Create a struct called `TestStruct` and serialize and deserialize it to and from JSON using the
 //! serialization API, using the derived serialization code.
 //!
-//! ```rust
+//! ```notrust
+//! // FIXME(#19470): this cannot be ```rust``` because it fails orphan checking at the moment
 //! extern crate serialize;
 //! use serialize::json;
 //!
@@ -110,7 +111,8 @@
 //!
 //! ### Simple example of `ToJson` usage
 //!
-//! ```rust
+//! ```notrust
+//! // FIXME(#19470): this cannot be ```rust``` because it fails orphan checking at the moment
 //! extern crate serialize;
 //! use serialize::json::{mod, ToJson, Json};
 //!
@@ -149,7 +151,8 @@
 //!
 //! ### Verbose example of `ToJson` usage
 //!
-//! ```rust
+//! ```notrust
+//! // FIXME(#19470): this cannot be ```rust``` because it fails orphan checking at the moment
 //! extern crate serialize;
 //! use std::collections::BTreeMap;
 //! use serialize::json::{mod, Json, ToJson};