Rollup merge of #104002 - RalfJung:unsafecell-new, r=JohnTitor

fix a comment in UnsafeCell::new

There are several safe methods that access the inner value: `into_inner` has existed since forever and `get_mut` also exists since recently. So this comment seems just wrong. But `&self` methods return raw pointers and thus require unsafe code (though the methods themselves are still safe).

library/core/src/cell.rs

@@ -1936,7 +1936,7 @@ impl<T> UnsafeCell<T> {
     /// Constructs a new instance of `UnsafeCell` which will wrap the specified
     /// value.
     ///
-    /// All access to the inner value through methods is `unsafe`.
+    /// All access to the inner value through `&UnsafeCell<T>` requires `unsafe` code.
     ///
     /// # Examples
     ///