Rollup merge of rust-lang#46148 - SimonSapin:patch-6, r=alexcrichton

Expand a couple points in 1.22.0 release notes

Let’s not give false hopes about `const fn` :)

RELEASES.md

@@ -5,9 +5,6 @@ Language
 --------
 - [`non_snake_case` lint now allows extern no-mangle functions][44966]
 - [Now accepts underscores in unicode escapes][43716]
-- [`#![feature(const_fn)]` is now no longer required for
-  calling const functions.][43017] It's still required for creating
-  constant functions.
 - [`T op= &T` now works for numeric types.][44287] eg. `let mut x = 2; x += &8;`
 - [types that impl `Drop` are now allowed in `const` and `static` types][44456]
 
@@ -45,8 +42,8 @@ Cargo
 Misc
 ----
 - [`libbacktrace` is now available on Apple platforms.][44251]
-- [Stabilised the `compile_fail` attribute for code fences.][43949] This now
-  lets you specify that a given code example will fail to compile.
+- [Stabilised the `compile_fail` attribute for code fences in doc-comments.][43949]
+  This now lets you specify that a given code example will fail to compile.
 
 Compatibility Notes
 -------------------