Stablize non_ascii_idents feature.

compiler/rustc_error_codes/src/error_codes/E0554.md

@@ -4,8 +4,8 @@ beta compilers will not comply.
 Erroneous code example:
 
 ```ignore (depends on release channel)
-#![feature(non_ascii_idents)] // error: `#![feature]` may not be used on the
-                              //        stable release channel
+#![feature(lang_items)] // error: `#![feature]` may not be used on the
+                        //        stable release channel
 ```
 
 If you need the feature, make sure to use a nightly release of the compiler

compiler/rustc_error_codes/src/error_codes/E0754.md

@@ -3,7 +3,6 @@ A non-ASCII identifier was used in an invalid context.
 Erroneous code examples:
 
 ```compile_fail,E0754
-# #![feature(non_ascii_idents)]
 
 mod řųśť; // error!
 
@@ -17,8 +16,6 @@ Non-ASCII can be used as module names if it is inlined or if a `#[path]`
 attribute is specified. For example:
 
 ```
-# #![feature(non_ascii_idents)]
-
 mod řųśť { // ok!
     const IS_GREAT: bool = true;
 }