Auto merge of rust-lang#52373 - eonil:master, r=kennytm

Update lint plugin sample code.

The sample code is out of sync with latest test code and broken.
I just copied missing feature gates.

Author: bors

src/doc/unstable-book/src/language-features/plugin.md

@@ -183,6 +183,8 @@ that warns about any item named `lintme`.
 ```rust,ignore
 #![feature(plugin_registrar)]
 #![feature(box_syntax, rustc_private)]
+#![feature(macro_vis_matcher)]
+#![feature(macro_at_most_once_rep)]
 
 extern crate syntax;