[skip e2e]Add comment for errIndexNodeIsUnhealthy (#15141)

Signed-off-by: cai.zhang <cai.zhang@zilliz.com>

internal/indexnode/errors.go

@@ -25,6 +25,7 @@ func msgIndexNodeIsUnhealthy(nodeID UniqueID) string {
 	return fmt.Sprintf("index node %d is not ready", nodeID)
 }
 
+// errIndexNodeIsUnhealthy return an error that specified IndexNode is not healthy.
 func errIndexNodeIsUnhealthy(nodeID UniqueID) error {
 	return errors.New(msgIndexNodeIsUnhealthy(nodeID))
 }