update "reason" for fnbox feature gate

It isn't "newly introduced" anymore.

Author: durka

src/liballoc/boxed.rs

@@ -525,14 +525,14 @@ impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for Box<I> {}
 /// }
 /// ```
 #[rustc_paren_sugar]
-#[unstable(feature = "fnbox", reason = "Newly introduced", issue = "28796")]
+#[unstable(feature = "fnbox", reason = "will be deprecated if and when Box<FnOnce> becomes usable", issue = "28796")]
 pub trait FnBox<A> {
     type Output;
 
     fn call_box(self: Box<Self>, args: A) -> Self::Output;
 }
 
-#[unstable(feature = "fnbox", reason = "Newly introduced", issue = "28796")]
+#[unstable(feature = "fnbox", reason = "will be deprecated if and when Box<FnOnce> becomes usable", issue = "28796")]
 impl<A, F> FnBox<A> for F where F: FnOnce<A>
 {
     type Output = F::Output;
@@ -542,7 +542,7 @@ impl<A, F> FnBox<A> for F where F: FnOnce<A>
     }
 }
 
-#[unstable(feature = "fnbox", reason = "Newly introduced", issue = "28796")]
+#[unstable(feature = "fnbox", reason = "will be deprecated if and when Box<FnOnce> becomes usable", issue = "28796")]
 impl<'a, A, R> FnOnce<A> for Box<FnBox<A, Output = R> + 'a> {
     type Output = R;
 
@@ -551,7 +551,7 @@ impl<'a, A, R> FnOnce<A> for Box<FnBox<A, Output = R> + 'a> {
     }
 }
 
-#[unstable(feature = "fnbox", reason = "Newly introduced", issue = "28796")]
+#[unstable(feature = "fnbox", reason = "will be deprecated if and when Box<FnOnce> becomes usable", issue = "28796")]
 impl<'a, A, R> FnOnce<A> for Box<FnBox<A, Output = R> + Send + 'a> {
     type Output = R;