[skip ci]Add comment for AssignSegmentID (#10319)

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

internal/distributed/datacoord/client/client.go

@@ -281,6 +281,19 @@ func (c *Client) Flush(ctx context.Context, req *datapb.FlushRequest) (*datapb.F
 	return ret.(*datapb.FlushResponse), err
 }
 
+// AssignSegmentID applies allocations for specified Coolection/Partition and related Channel Name(Virtial Channel)
+//
+// ctx is the context to control request deadline and cancellation
+// req contains the requester's info(id and role) and the list of Assignment Request,
+// which coontains the specified collection, partitaion id, the related VChannel Name and row count it needs
+//
+// response struct `AssignSegmentIDResponse` contains the the assignment result for each request
+// error is returned only when some communication issue occurs
+// if some error occurs in the process of `AssignSegmentID`, it will be recorded and returned in `Status` field of response
+//
+// `AssignSegmentID` will applies current configured allocation policies for each request
+// if the VChannel is newly used, `WatchDmlChannels` will be invoked to notify a `DataNode`(selected by policy) to watch it
+// if there is anything make the allocation impossible, the response will not contain the corresponding result
 func (c *Client) AssignSegmentID(ctx context.Context, req *datapb.AssignSegmentIDRequest) (*datapb.AssignSegmentIDResponse, error) {
 	ret, err := c.recall(func() (interface{}, error) {
 		client, err := c.getGrpcClient()