restructure module documentation order

split hiearchy into one top level module per library
diff --git a/lib/msg.c b/lib/msg.c
index d08d057..22761a0 100644
--- a/lib/msg.c
+++ b/lib/msg.c
@@ -10,7 +10,7 @@
  */
 
 /**
- * @ingroup nl
+ * @ingroup core
  * @defgroup msg Messages
  * Netlink Message Construction/Parsing Interface
  *