Add comment for boolean literal function argument

Co-authored-by: Jon Ross-Perkins <[email protected]>

toolchain/check/handle_class.cpp

@@ -725,7 +725,7 @@ static auto CheckCompleteClassType(Context& context, Parse::NodeId node_id,
             // one. The functions should be allowed to have different signatures
             // as long as we can synthesize a suitable thunk.
             CheckFunctionTypeMatches(context, override_fn, fn,
-                                     SemIR::SpecificId::Invalid, false);
+                                     SemIR::SpecificId::Invalid, /*check_syntax=*/false);
             fn_decl_id = override_fn_decl_id;
           }
         }