[skip e2e]Add comment for msgIndexCoordIsUnhealthy (#15146)

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

internal/indexcoord/errors.go

@@ -26,6 +26,7 @@ func errIndexNodeIsNotOnService(id UniqueID) error {
 	return fmt.Errorf("index node %d is not on service", id)
 }
 
+// msgIndexCoordIsUnhealthy return an error that the IndexCoord is not healthy.
 func msgIndexCoordIsUnhealthy(coordID UniqueID) string {
 	return fmt.Sprintf("IndexCoord %d is not ready", coordID)
 }