[skip e2e]Add comment for errIndexNodeIsNotOnService (#14986)

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

internal/indexcoord/errors.go

@@ -21,6 +21,7 @@ import (
 	"fmt"
 )
 
+// errIndexNodeIsNotOnService return an error that the specified IndexNode is not exists.
 func errIndexNodeIsNotOnService(id UniqueID) error {
 	return fmt.Errorf("index node %d is not on service", id)
 }