Revise wording in Rc documentation.

The term "thread-local" has a widely-accepted meaning which is not
the meaning it's used for here.

Author: Sam Payson

src/liballoc/rc.rs

@@ -10,7 +10,7 @@
 
 #![allow(deprecated)]
 
-//! Thread-local reference-counted boxes (the `Rc<T>` type).
+//! Unsynchronized reference-counted boxes (the `Rc<T>` type).
 //!
 //! The `Rc<T>` type provides shared ownership of an immutable value.
 //! Destruction is deterministic, and will occur as soon as the last owner is