rustdoc: Move display_fn struct inside display_fn

This makes it clear that it's an implementation detail of `display_fn`
and shouldn't be used elsewhere, and it enforces in the compiler that no
one else can use it.

src/librustdoc/html/format.rs

@@ -1322,9 +1322,6 @@ impl clean::GenericArg {
 }
 
 crate fn display_fn(f: impl FnOnce(&mut fmt::Formatter<'_>) -> fmt::Result) -> impl fmt::Display {
-    WithFormatter(Cell::new(Some(f)))
-}
-
     struct WithFormatter<F>(Cell<Option<F>>);
 
     impl<F> fmt::Display for WithFormatter<F>
@@ -1335,3 +1332,6 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
             (self.0.take()).unwrap()(f)
         }
     }
+
+    WithFormatter(Cell::new(Some(f)))
+}