Rollup merge of rust-lang#34097 - arbitrary-cat:master, r=steveklabnik

Revise wording in Rc documentation.

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

src/liballoc/rc.rs

@@ -10,7 +10,8 @@
 
 #![allow(deprecated)]
 
-//! Thread-local reference-counted boxes (the `Rc<T>` type).
+//! Unsynchronized reference-counted boxes (the `Rc<T>` type) which are usable
+//! only within a single thread.
 //!
 //! The `Rc<T>` type provides shared ownership of an immutable value.
 //! Destruction is deterministic, and will occur as soon as the last owner is