ndp: sixlowpan_nd: explain maximum retransmission for DAD

In #8277 it was noted the doc of general ND definitions refers to
GNRC-specific behavior. This is now clarified with this fix.

sys/include/net/ndp.h

@@ -157,7 +157,9 @@ extern "C" {
 /**
  * @brief   Default for DupAddrDetectTransmits
  * @see     [RFC 4862, section 5.1](https://tools.ietf.org/html/rfc4862#section-5.1)
- * @note    Must not be greater than 7.
+ * @note    Must not be greater than 7 for @ref net_gnrc since
+ *          @ref GNRC_NETIF_IPV6_ADDRS_FLAGS_STATE_TENTATIVE restricts it to
+ *          that number.
  */
 #define NDP_DAD_TRANSMIT_NUMOF      (1U)
 #define NDP_MAX_ANYCAST_MS_DELAY    (1000U)     /**< MAX_ANYCAST_DELAY_TIME (in ms) */

sys/include/net/sixlowpan/nd.h

@@ -115,7 +115,9 @@ extern "C" {
 /**
  * @brief   Number of address registration retries
  *
- * @note    Must not be greater than 7.
+ * @note    Must not be greater than 7 for @ref net_gnrc since
+ *          @ref GNRC_NETIF_IPV6_ADDRS_FLAGS_STATE_TENTATIVE restricts it to
+ *          that number.
  */
 #define SIXLOWPAN_ND_REG_TRANSMIT_NUMOF         (3U)
 /**