lint: port non-existant doc keyword diagnostics

Signed-off-by: David Wood <david.wood@huawei.com>

compiler/rustc_error_messages/locales/en-US/lint.ftl

@@ -46,3 +46,6 @@ lint-ty-qualified = usage of qualified `ty::{$ty}`
 
 lint-lintpass-by-hand = implementing `LintPass` by hand
     .help = try using `declare_lint_pass!` or `impl_lint_pass!` instead
+
+lint-non-existant-doc-keyword = found non-existing keyword `{$keyword}` used in `#[doc(keyword = \"...\")]`
+    .help = only existing keywords are allowed in core/std

compiler/rustc_lint/src/internal.rs

@@ -366,13 +366,10 @@ impl<'tcx> LateLintPass<'tcx> for ExistingDocKeyword {
                             return;
                         }
                         cx.struct_span_lint(EXISTING_DOC_KEYWORD, attr.span, |lint| {
-                            lint.build(&format!(
-                                "Found non-existing keyword `{}` used in \
-                                     `#[doc(keyword = \"...\")]`",
-                                v,
-                            ))
-                            .help("only existing keywords are allowed in core/std")
-                            .emit();
+                            lint.build(fluent::lint::non_existant_doc_keyword)
+                                .set_arg("keyword", v)
+                                .help(fluent::lint::help)
+                                .emit();
                         });
                     }
                 }

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

@@ -1,4 +1,4 @@
-error: Found non-existing keyword `tadam` used in `#[doc(keyword = "...")]`
+error: found non-existing keyword `tadam` used in `#[doc(keyword = \"...\")]`
   --> $DIR/existing_doc_keyword.rs:10:1
    |
 LL | #[doc(keyword = "tadam")]