Extend test to ensure that items inherit lint level from the parent

Author: GuillaumeGomez

src/test/rustdoc-ui/coverage/allow_missing_docs.rs

@@ -36,4 +36,6 @@ pub mod bar {
     pub struct Bar { //~ WARN
         pub f: u32, //~ WARN
     }
+
+    pub struct NeedsNoDocs;
 }