adjust the document to the sad reality

Author: RalfJung

promotion.md

@@ -98,8 +98,9 @@ promotion.
 Currently, the following are considered explicit promotion contexts:
 * `#[rustc_args_required_const]` arguments
 * lifetime extension in the bodies of `const` and `static` items and array lengths
+* the bodies of `const fn` -- but this is not justified by the above discussion; since `const fn` can be runtime-evaluated, the concerns about introducing hard errors where there would be none apply for `const fn` in exactly the same way as for `fn`, but this was overlooked when implementing the checks
 
-Everything else is an implicit promotion context, including `const fn` bodies and non-`Copy` array initializers.
+Everything else is an implicit promotion context, in particular non-`Copy` array initializers.
 
 In an explicit promotion context, we consider almost everything promotable,
 subject to the usual restrictions of [consts](const.md). This means that