Fix outdated comment next to array_impl_default

The comment has become outdated as the array_impl macro
has been removed.

library/core/src/array/mod.rs

@@ -353,8 +353,9 @@ impl<T: Ord, const N: usize> Ord for [T; N] {
     }
 }
 
-// The Default impls cannot be generated using the array_impls! macro because
+// The Default impls cannot be done with const generics because `[T; 0]` doesn't
-// they require array literals.
+// require Default to be implemented, and having different impl blocks for
+// different numbers isn't supported yet.
 
 macro_rules! array_impl_default {
     {$n:expr, $t:ident $($ts:ident)*} => {