Rollup merge of #27326 - steveklabnik:doc_show_use, r=Gankro

In spirit with https://internals.rust-lang.org/t/should-we-keep-including-obvious-imports-in-code-examples/2217, show the feature flags we're using in examples.

(also one instance of 'use')

src/libstd/ascii.rs

@@ -125,7 +125,8 @@ pub trait AsciiExt {
     /// # Examples
     ///
     /// ```
-    /// # #![feature(ascii)]
+    /// #![feature(ascii)]
+    ///
     /// use std::ascii::AsciiExt;
     ///
     /// let mut ascii = 'a';
@@ -144,7 +145,8 @@ pub trait AsciiExt {
     /// # Examples
     ///
     /// ```
-    /// # #![feature(ascii)]
+    /// #![feature(ascii)]
+    ///
     /// use std::ascii::AsciiExt;
     ///
     /// let mut ascii = 'A';