Make more doc attribute parsing error into future warnings

compiler/rustc_lint/messages.ftl

@@ -558,6 +558,10 @@ lint_macro_expr_fragment_specifier_2024_migration =
 
 lint_malformed_attribute = malformed lint attribute input
 
+lint_malformed_doc =
+    malformed `doc` attribute input
+    .warn = {-lint_previously_accepted}
+
 lint_map_unit_fn = `Iterator::map` call that discard the iterator's values
     .note = `Iterator::map`, like many of the methods on `Iterator`, gets executed lazily, meaning that its effects won't be visible until it is iterated
     .function_label = this function returns `()`, which is likely not what you wanted

compiler/rustc_lint/src/early/diagnostics.rs

@@ -428,5 +428,7 @@ pub fn decorate_attribute_lint(
             sugg: suggested.map(|s| lints::UnknownCrateTypesSuggestion { span, snippet: s }),
         }
         .decorate_lint(diag),
+
+        &AttributeLintKind::MalformedDoc => lints::MalformedDoc.decorate_lint(diag),
     }
 }

compiler/rustc_lint/src/lints.rs

@@ -3185,6 +3185,11 @@ pub(crate) struct UnusedDuplicate {
     pub warning: bool,
 }
 
+#[derive(LintDiagnostic)]
+#[diag(lint_malformed_doc)]
+#[warning]
+pub(crate) struct MalformedDoc;
+
 #[derive(LintDiagnostic)]
 #[diag(lint_unsafe_attr_outside_unsafe)]
 pub(crate) struct UnsafeAttrOutsideUnsafeLint {