Merge pull request rust-lang#1596 from folkertdev/stabilize-const-extern-fn

stabilize `const_extern_fn`

Author: ehuss

src/items/functions.md

@@ -227,7 +227,7 @@ Functions qualified with the `const` keyword are [const functions], as are
 [tuple struct] and [tuple variant] constructors. _Const functions_  can be
 called from within [const contexts].
 
-Const functions may use the [`extern`] function qualifier, but only with the `"Rust"` and `"C"` ABIs.
+Const functions may use the [`extern`] function qualifier.
 
 Const functions are not allowed to be [async](#async-functions).