doxygen: add @file declarations for src/feature/

If a file doesn't use the file command (either \file or @file),
Doxygen won't try to process it.

src/feature/dirparse/unparseable.c

@@ -4,6 +4,11 @@
  * Copyright (c) 2007-2019, The Tor Project, Inc. */
 /* See LICENSE for licensing information */
 
+/**
+ * @file unparseable.c
+ * @brief Dump unparseable objects to disk.
+ **/
+
 #define UNPARSEABLE_PRIVATE
 
 #include "core/or/or.h"