docs(unstable): Make the nightly link standout a bit more

Author: epage

src/doc/src/reference/unstable.md

@@ -52,7 +52,8 @@ how the feature works:
   ```
 
 Each new feature described below should explain how to use it.
-For the latest nightly, see the [nightly version] of this page.
+
+*For the latest nightly, see the [nightly version] of this page.*
 
 [config file]: config.md
 [nightly channel]: ../../book/appendix-07-nightly-rust.html