Cm[ox]_format: document fields that are stored in reverse order.

Author: dbuenzli

file_formats/cmo_format.mli

@@ -53,6 +53,8 @@ type compilation_unit =
 type library =
   { lib_units: compilation_unit list;   (* List of compilation units *)
     lib_custom: bool;                   (* Requires custom mode linking? *)
+    (* In the following fields the lists are reversed with respect to
+       how they end up being used on the command line. *)
     lib_ccobjs: string list;            (* C object files needed for -custom *)
     lib_ccopts: string list;            (* Extra opts to C compiler *)
     lib_dllibs: string list }           (* DLLs needed *)

file_formats/cmx_format.mli

@@ -52,5 +52,7 @@ type unit_infos =
 
 type library_infos =
   { lib_units: (unit_infos * Digest.t) list;  (* List of unit infos w/ MD5s *)
+    (* In the following fields the lists are reversed with respect to
+       how they end up being used on the command line. *)
     lib_ccobjs: string list;            (* C object files needed *)
     lib_ccopts: string list }           (* Extra opts to C compiler *)