Add feature gate test for "rustdoc_internal" feature

Author: GuillaumeGomez

src/test/ui-fulldeps/internal-lints/existing_doc_keyword.rs

@@ -1,7 +1,7 @@
 // compile-flags: -Z unstable-options
 
 #![feature(rustc_private)]
-#![feature(doc_keyword)]
+#![feature(rustdoc_internals)]
 
 #![crate_type = "lib"]
 

src/test/ui/feature-gates/feature-gate-rustdoc_internals.rs

@@ -0,0 +1,5 @@
+#[doc(keyword = "match")] //~ ERROR: `#[doc(keyword)]` is experimental
+/// wonderful
+mod foo {}
+
+fn main() {}

src/test/ui/feature-gates/feature-gate-rustdoc_internals.stderr

@@ -0,0 +1,12 @@
+error[E0658]: `#[doc(keyword)]` is experimental
+  --> $DIR/feature-gate-rustdoc_internals.rs:1:1
+   |
+LL | #[doc(keyword = "match")]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #90418 <https://github.com/rust-lang/rust/issues/90418> for more information
+   = help: add `#![feature(rustdoc_internals)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.