[skip ci] Add comment for queryCoord (#9266)

Signed-off-by: Nxige-16 <xi.ge@zilliz.com>

internal/querycoord/query_coord.go

@@ -45,6 +45,7 @@ type queryChannelInfo struct {
 	responseChannel string
 }
 
+// QueryCoord is the coordinator of queryNodes
 type QueryCoord struct {
 	loopCtx    context.Context
 	loopCancel context.CancelFunc