Add a brief overview of each file to the docs.

strerror_override_private.h

@@ -1,6 +1,11 @@
 #ifndef __json_strerror_override_private_h__
 #define __json_strerror_override_private_h__
 
+/**
+ * @file
+ * @brief Do not use, json-c internal, may be changed or removed at any time.
+ */
+
 /* Used by tests to get consistent output */
 extern int _json_c_strerror_enable;