[skip ci]Add comment for SaveBinlogPaths (#10777)

Signed-off-by: Nsunby <bingyi.sun@zilliz.com>

internal/distributed/datacoord/client/client.go

@@ -441,6 +441,18 @@ func (c *Client) GetSegmentInfo(ctx context.Context, req *datapb.GetSegmentInfoR
 	return ret.(*datapb.GetSegmentInfoResponse), err
 }
 
+// SaveBinlogPaths updates segments binlogs(including insert binlogs, stats logs and delta logs)
+//  and related message stream positions
+//
+// ctx is the context to control request deadline and cancellation
+// req contains the collection/partition id to query
+//
+// response status contains the status/error code and failing reason if any
+// error is returned only when some communication issue occurs
+//
+// there is a constraint that the `SaveBinlogPaths` requests of same segment shall be passed in sequence
+// 	the root reason is each `SaveBinlogPaths` will overwrite the checkpoint position
+//  if the constraint is broken, the checkpoint position will not be monotonically increasing and the integrity will be compromised
 func (c *Client) SaveBinlogPaths(ctx context.Context, req *datapb.SaveBinlogPathsRequest) (*commonpb.Status, error) {
 	// FIXME(dragondriver): why not to recall here?
 	client, err := c.getGrpcClient()