Add a brief overview of each file to the docs.

strdup_compat.h

@@ -1,6 +1,11 @@
 #ifndef __strdup_compat_h
 #define __strdup_compat_h
 
+/**
+ * @file
+ * @brief Do not use, json-c internal, may be changed or removed at any time.
+ */
+
 #if !defined(HAVE_STRDUP) && defined(_MSC_VER)
   /* MSC has the version as _strdup */
 # define strdup _strdup