Lora: Improve description of callback methods

Users often use "show declaration" (or similar) in their IDE to see descriptions of callback
methods so the function descriptions should be placed close to function declaration itself.

features/lorawan/lorawan_types.h

@@ -242,10 +242,6 @@ typedef enum lora_events {
  * 'link_check_resp' callback. The result is thus transported to the application
  * via callback function provided.
  *
- * As can be seen from declaration, mbed::Callback<void(uint8_t, uint8_t)> *link_check_resp)
- * carries two parameters. First one is Demodulation Margin and the second one
- * is number of gateways involved in the path to network server.
- *
  * 'battery_level' callback goes in the down direction, i.e., it informs
  * the stack about the battery level by calling a function provided
  * by the upper layers.
@@ -257,11 +253,16 @@ typedef struct {
     mbed::Callback<void(lorawan_event_t)> events;
 
     /**
-     * Optional
+     * This callback is optional
+     *
+     * The first parameter to callback function is the demodulation margin, and the second
+     * parameter is the number of gateways that successfully received the last request.
      */
     mbed::Callback<void(uint8_t, uint8_t)> link_check_resp;
 
     /**
+     * This callback is optional. If callback is not set, stack will return 255 to gateway.
+     *
      * Battery level return value must follow the specification
      * for DevStatusAns MAC command:
      *