nimble/ll: Add note about InitA when initiating connection with privacy

Even with privacy enabled we have to use our identity address as InitA
when peer is not on our resolving list. Since this is a bit counter-
intuitive, let's add a note with spec reference so we do not need to
waste time in future figuring out why it is implemented like this.

X-Original-Commit: 19cf6a3f92b9111f05fcd2f05cfa21bb62991cb9

nimble/controller/src/ble_ll_conn.c

@@ -2728,6 +2728,11 @@ ble_ll_conn_req_pdu_update(struct os_mbuf *m, uint8_t *adva, uint8_t addr_type,
             }
         }
 
+        /*
+         * If peer in on resolving list, we use RPA generated with Local IRK
+         * from resolving list entry. In other case, we need to use our identity
+         * address (see  Core 5.0, Vol 6, Part B, section 6.4).
+         */
         if (rl) {
             hdr |= BLE_ADV_PDU_HDR_TXADD_RAND;
             ble_ll_resolv_gen_priv_addr(rl, 1, dptr);