Fix clippy and rustdoc

please, please, don't match on `Symbol::as_str`s, every time you do,
somewhere in the world another waffle becomes sad...

compiler/rustc_span/src/symbol.rs

@@ -542,6 +542,7 @@ symbols! {
         core,
         core_panic,
         core_panic_2015_macro,
+        core_panic_2021_macro,
         core_panic_macro,
         cosf32,
         cosf64,
@@ -621,6 +622,7 @@ symbols! {
         doc_primitive,
         doc_spotlight,
         doctest,
+        document_private_items,
         dotdot: "..",
         dotdot_in_tuple_patterns,
         dotdoteq_in_patterns,