Update notes with regards to feature gates

* `const_transmute` currently also seems to depend on the `const_fn`
  feature.
* Only `Sized` is currently allowed as a bound, not Copy.

Author: phansch

tests/ui/missing_const_for_fn/could_be_const.rs

@@ -41,12 +41,14 @@ fn generic<T>(t: T) -> T {
     t
 }
 
-// FIXME: This could be const but is currently not linted
+// FIXME: Depends on the `const_transmute` and `const_fn` feature gates.
+// In the future Clippy should be able to suggest this as const, too.
 fn sub(x: u32) -> usize {
     unsafe { transmute(&x) }
 }
 
-// FIXME: This could be const but is currently not linted
+// NOTE: This is currently not yet allowed to be const
+// Once implemented, Clippy should be able to suggest this as const, too.
 fn generic_arr<T: Copy>(t: [T; 1]) -> T {
     t[0]
 }