rustdoc: Don't strip empty modules with documentation

Closes #11443

src/librustdoc/passes.rs

@@ -136,7 +136,9 @@ impl<'a> fold::DocFolder for Stripper<'a> {
             Some(i) => {
                 match i.inner {
                     // emptied modules/impls have no need to exist
-                    clean::ModuleItem(ref m) if m.items.len() == 0 => None,
+                    clean::ModuleItem(ref m)
+                        if m.items.len() == 0 &&
+                           i.doc_value().is_none() => None,
                     clean::ImplItem(ref i) if i.methods.len() == 0 => None,
                     _ => {
                         self.retained.insert(i.id);