Add rustdoc-ui tests for warning emitted when auto_cfg is not used with doc_auto_cfg feature

Author: GuillaumeGomez

src/test/rustdoc-ui/doc_auto_cfg.rs

@@ -0,0 +1,2 @@
+// check-pass
+#![feature(doc_auto_cfg)]

src/test/rustdoc-ui/doc_auto_cfg.stderr

@@ -0,0 +1,4 @@
+warning: the `doc_auto_cfg` feature is used but `#![doc(auto_cfg)]` isn't so it won't do anything
+
+warning: 1 warning emitted
+

src/test/rustdoc-ui/doc_no_auto_cfg.stderr

@@ -4,15 +4,15 @@ error: this attribute can only be applied at the crate level
 LL | #[doc(auto_cfg)]
    |       ^^^^^^^^
    |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #82730 <https://github.com/rust-lang/rust/issues/82730>
+   = note: read <https://doc.rust-lang.org/nightly/rustdoc/the-doc-attribute.html#at-the-crate-level> for more information
 note: the lint level is defined here
   --> $DIR/doc_no_auto_cfg.rs:2:9
    |
 LL | #![deny(warnings)]
    |         ^^^^^^^^
    = note: `#[deny(invalid_doc_attributes)]` implied by `#[deny(warnings)]`
-   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #82730 <https://github.com/rust-lang/rust/issues/82730>
-   = note: read <https://doc.rust-lang.org/nightly/rustdoc/the-doc-attribute.html#at-the-crate-level> for more information
 help: to apply to the crate, use an inner attribute
    |
 LL | #![doc(auto_cfg)]