doc: Replace doc_auto_cfg by doc_cfg

doc_auto_cfg has been merged into doc_cfg:

error[E0557]: feature has been removed
 --> src/lib.rs:2:40
  |
2 | #![cfg_attr(all(doc, nightly), feature(doc_auto_cfg))] // expect nightly for doc
  |                                        ^^^^^^^^^^^^ feature has been removed
  |
  = note: removed in CURRENT_RUSTC_VERSION; see <rust-lang/rust#138907> for more information
  = note: merged into `doc_cfg`

Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
(cherry picked from commit 273e6a2)
Signed-off-by: Hanna Czenczek <hreitz@redhat.com>

Author: XanClic

src/lib.rs

@@ -1,5 +1,5 @@
 // #![feature(async_drop)] -- enable with async-drop
-#![cfg_attr(all(doc, nightly), feature(doc_auto_cfg))] // expect nightly for doc
+#![cfg_attr(all(doc, nightly), feature(doc_cfg))] // expect nightly for doc
 #![warn(missing_docs)]
 #![warn(clippy::missing_docs_in_private_items)]