clarify the role of rustc_const_unstable

Author: RalfJung

src/stability.md

@@ -51,9 +51,9 @@ marks an item as stabilized. Note that stable functions may use unstable things
 
 The `#[rustc_const_unstable(feature = "foo", issue = "1234", reason = "lorem ipsum")]`
 has the same interface as the `unstable` attribute. It is used to mark
-`const fn` as having their constness be unstable. This allows you to make a
-function stable without stabilizing its constness or even just marking an existing
-stable function as `const fn` without instantly stabilizing the `const fn`ness.
+`const fn` as having their constness be unstable. Every function that is eventually intended
+to become const-stable should carry this attribute, no matter whether it is `#[stable]`
+or `#[unstable]`.
 
 Furthermore this attribute is needed to mark an intrinsic as `const fn`, because
 there's no way to add `const` to functions in `extern` blocks for now.