[docs] More explicit description of grpdata_size updates (#2820).

docs/API/API-functions.md

@@ -1832,7 +1832,8 @@ call [`srt_sendmsg2`](#srt_sendmsg) or [`srt_recvmsg2`](#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 the [SRT Connection Bonding: Quick Start](../features/bonding-intro.md) and
+the array; otherwise both fields are updated to reflect the current connection state
+of the group. For details, see the [SRT Connection Bonding: Quick Start](../features/bonding-intro.md) and
 [SRT Connection Bonding: Socket Groups](../features/socket-groups.md) documents.
 
 **Helpers for [`SRT_MSGCTRL`](#SRT_MSGCTRL):**