Add a brief overview of each file to the docs.

math_compat.h

@@ -1,6 +1,11 @@
 #ifndef __math_compat_h
 #define __math_compat_h
 
+/**
+ * @file
+ * @brief Do not use, json-c internal, may be changed or removed at any time.
+ */
+
 /* Define isnan, isinf, infinity and nan on Windows/MSVC */
 
 #ifndef HAVE_DECL_ISNAN