Update error message for doc(keyword)

Author: GuillaumeGomez

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -328,7 +328,7 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                 );
 
                 if nested_meta.has_name(sym::keyword) {
-                    let msg = concat!("`#[doc(keyword)]` is meant for internal use only");
+                    let msg = "`#[doc(keyword)]` is meant for internal use only";
                     gate_feature_post!(self, rustdoc_internals, attr.span, msg);
                 }
             }

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

@@ -1,4 +1,4 @@
-#[doc(keyword = "match")] //~ ERROR: `#[doc(keyword)]` is experimental
+#[doc(keyword = "match")] //~ ERROR: `#[doc(keyword)]` is meant for internal use only
 /// wonderful
 mod foo {}
 

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

@@ -1,4 +1,4 @@
-error[E0658]: `#[doc(keyword)]` is experimental
+error[E0658]: `#[doc(keyword)]` is meant for internal use only
   --> $DIR/feature-gate-rustdoc_internals.rs:1:1
    |
 LL | #[doc(keyword = "match")]