Minor (NGAP): clarify ngap_register_gnb_req_t actually refers to NG SETUP REQUEST

There is no such thing as NG Register message, the naming is
inaccurate, however the goal of this commit is only to clarify that
and major refactoring is not yet carried out.

openair2/COMMON/ngap_messages_types.h

@@ -351,7 +351,11 @@ typedef struct {
 } ngap_plmn_t;
 
 //-------------------------------------------------------------------------------------------//
-// gNB application layer -> NGAP messages
+
+/** @brief NG SETUP REQUEST message (9.2.6.1 of 3GPP TS 38.413)
+ * This message is sent by the NG-RAN node to transfer application layer information
+ * for an NG-C interface instance.
+ * Direction: NG-RAN node -> AMF */
 typedef struct ngap_register_gnb_req_s {
   /* Unique gNB_id to identify the gNB within EPC.
    * For macro gNB ids this field should be 20 bits long.