add doc

Author: mengxr

mllib/src/main/scala/org/apache/spark/ml/feature/StringIndexer.scala

@@ -88,6 +88,9 @@ class StringIndexer(override val uid: String) extends Estimator[StringIndexerMod
 /**
  * :: Experimental ::
  * Model fitted by [[StringIndexer]].
+ * NOTE: During transformation, if the input column does not exist,
+ * [[StringIndexerModel.transform]] would return the input dataset unmodified.
+ * This is a temporary fix for the case when target labels do not exist during prediction.
  */
 @Experimental
 class StringIndexerModel private[ml] (