Remove doc of unstable feature of never type

Author: hehaoqian

src/types/never.md

@@ -5,18 +5,19 @@
 
 The never type `!` is a type with no values, representing the result of
 computations that never complete. Expressions of type `!` can be coerced into
-any other type.
+any other type. 
 
-<!-- ignore: unstable -->
-```rust,ignore
-let x: ! = panic!();
-// Can be coerced into any type.
-let y: u32 = x;
+The `!` type can **only** appear in function return types presently,
+indicating it is a diverging function that never returns.
+
+```rust
+fn foo() -> ! {
+    panic!("This call never returns.");
+}
 ```
 
-**NB.** The never type was expected to be stabilized in 1.41, but due
-to some last minute regressions detected the stabilization was
-temporarily reverted. The `!` type can only appear in function return
-types presently. See [the tracking
-issue](https://github.com/rust-lang/rust/issues/35121) for more
-details.
+```rust
+extern "C" {
+    pub fn no_return_extern_func() -> !;
+}
+```