net/coap: use separate name for macro doc

sys/include/net/coap.h

@@ -162,6 +162,10 @@ extern "C" {
  * @ingroup  config
  * @{
  */
+/**
+ * @name    Timing parameters
+ * @{
+ */
 /**
  * @brief    Timeout in seconds for a response to a confirmable request
  *
@@ -198,9 +202,10 @@ extern "C" {
 #define COAP_MAX_RETRANSMIT     (4)
 #endif
 /** @} */
+/** @} */
 
 /**
- * @name    Timing parameters
+ * @name    Fixed timing parameters
  * @{
  */
 #define COAP_NSTART             (1)