Update const-fn doc in unstable-book

Update src/doc/unstable-book/src/language-features/const-fn.md

Co-authored-by: Ivan Tham <[email protected]>

Author: sasurau4

src/doc/unstable-book/src/language-features/const-fn.md

@@ -6,24 +6,5 @@ The tracking issue for this feature is: [#57563]
 
 ------------------------
 
-The `const_fn` feature allows marking free functions and inherent methods as
-`const`, enabling them to be called in constants contexts, with constant
-arguments.
-
-## Examples
-
-```rust
-#![feature(const_fn)]
-
-const fn double(x: i32) -> i32 {
-    x * 2
-}
-
-const FIVE: i32 = 5;
-const TEN: i32 = double(FIVE);
-
-fn main() {
-    assert_eq!(5, FIVE);
-    assert_eq!(10, TEN);
-}
-```
+The `const_fn` feature enables additional functionality not stabilized in the
+[minimal subset of `const_fn`](https://github.com/rust-lang/rust/issues/53555)