hide alloc::alloc::box_new in docs

It's an internal function which isn't supposed to be used outside
standard library / `vec!`.

Author: WaffleLapkin

alloc/src/boxed.rs

@@ -237,6 +237,7 @@ pub struct Box<
 /// the newly allocated memory. This is an intrinsic to avoid unnecessary copies.
 ///
 /// This is the surface syntax for `box <expr>` expressions.
+#[doc(hidden)]
 #[rustc_intrinsic]
 #[unstable(feature = "liballoc_internals", issue = "none")]
 pub fn box_new<T>(x: T) -> Box<T>;