Make cfg implicitly imply doc(cfg)

This is only active when the `doc_cfg` feature is active.

The implicit cfg can be overridden via #[doc(cfg(...))], so e.g. to
hide a #[cfg] you can use something like:

```rust
 #[cfg(unix)]
 #[doc(cfg(all()))]
pub struct Unix;
```

(since `all()` is always true, it is never shown in the docs)

src/librustdoc/html/render/print_item.rs

@@ -323,7 +323,7 @@ fn item_module(w: &mut Buffer, cx: &Context<'_>, item: &clean::Item, items: &[cl
                     let import_item = clean::Item {
                         def_id: import_def_id.into(),
                         attrs: import_attrs,
-                        cfg: ast_attrs.cfg(cx.sess()),
+                        cfg: ast_attrs.cfg(cx.tcx()),
                         ..myitem.clone()
                     };