Update cell.rs, clarified Cell<T> in module level doc

Specifically, removed statement regarding at most one references to inner value, as Cell<T>'s interface is designed to prevent the existence of any reference to Cell<T>'s inner value.

Author: matwatson

library/core/src/cell.rs

@@ -28,9 +28,8 @@
 //! ## `Cell<T>`
 //!
 //! [`Cell<T>`] implements interior mutability by moving values in and out of the cell. That is, an
-//! `& T` to the inner value can never be obtained, and the value itself cannot be directly
-//! obtained without replacing it with something else. Both of these rules ensure that there is
-//! never more than one mutable reference pointing to the inner value. This type provides the following
+//! `&T` to the inner value can never be obtained, and the value itself cannot be directly
+//! obtained without replacing it with something else. This type provides the following
 //! methods:
 //!
 //!  - For types that implement [`Copy`], the [`get`](Cell::get) method retrieves the current