[docs] Added lacking description for grpdata field in SRT_MSGCTRL (#1623

)

docs/API-functions.md

@@ -1231,6 +1231,8 @@ typedef struct SRT_MsgCtrl_
    int64_t srctime;      // source time (microseconds since SRT internal clock epoch)
    int32_t pktseq;       // sequence number of the first packet in received message (unused for sending)
    int32_t msgno;        // message number (output value for both sending and receiving)
+   SRT_SOCKGROUPDATA* grpdata; // pointer to group data array
+   size_t grpdata_size;  // size of the group array
 } SRT_MSGCTRL;
 ```
 
@@ -1277,6 +1279,15 @@ UDP packet, only the sequence of the first one is reported. Note that in
 although it is required that this value remain monotonic in subsequent send calls. 
 Normally message numbers start with 1 and increase with every message sent.
 
+- 'grpdata' and 'grpdata_size': Pointer and size of the group array. For single
+socket connections these values should remain NULL and 0 respectively. When you
+call `srt_sendmsg2` or `srt_recvmsg2` function for a group, you should pass an
+array here so that you can retrieve the status of particular member sockets.
+If you pass an array that is too small, your `grpdata_size` field will be rewritten with
+the current number of members, but without filling in the array. For details,
+see (Bonding introduction)[bonding-intro.md] and (Socket Groups)[socket-groups.md]
+documents.
+
 **Helpers for `SRT_MSGCTRL`:**
 
 ```