Add Once::new as a way of constructing a Once

Author: tbu-

src/libstd/sync/once.rs

@@ -73,9 +73,10 @@ use thread::{self, Thread};
 /// A synchronization primitive which can be used to run a one-time global
 /// initialization. Useful for one-time initialization for FFI or related
 /// functionality. This type can only be constructed with the [`ONCE_INIT`]
-/// value.
+/// value or the equivalent [`Once::new`] constructor.
 ///
 /// [`ONCE_INIT`]: constant.ONCE_INIT.html
+/// [`Once::new`]: struct.Once.html#method.new
 ///
 /// # Examples
 ///