Rollup merge of #85280 - jsha:move-trait-toggles, r=GuillaumeGomez

Toggle-wrap items differently than top-doc.

This makes sure things like trait methods get wrapped at the
`<h3><code>` level rather than at the `.docblock` level. Also it ensures
that only the actual top documentation gets the `.top-doc` class.

Fixes #85167

Before:

![image](https://user-images.githubusercontent.com/220205/117743384-98790200-b1bb-11eb-8804-588530842514.png)

https://doc.rust-lang.org/nightly/std/io/trait.Read.html#tymethod.read

After:

![image](https://user-images.githubusercontent.com/220205/118410882-98a75080-b646-11eb-949d-ca688bab6923.png)

src/test/rustdoc/toggle-trait-fn.rs

@@ -0,0 +1,7 @@
+#![crate_name = "foo"]
+
+// @has foo/trait.Foo.html
+// @has - '//details[@class="rustdoc-toggle"]//code' 'bar'
+pub trait Foo {
+    fn bar() -> ();
+}