update document

Signed-off-by: VinceShieh <vincent.xie@intel.com>

Author: VinceShieh

docs/ml-features.md

@@ -502,7 +502,7 @@ for more details on the API.
 ## StringIndexer
 
 `StringIndexer` encodes a string column of labels to a column of label indices.
-The indices are in `[0, numLabels)`, ordered by label frequencies, so the most frequent label gets index `0`.
+The indices are in `[0, numLabels]`, ordered by label frequencies, so the most frequent label gets index `0`.
 If the input column is numeric, we cast it to string and index the string
 values. When downstream pipeline components such as `Estimator` or
 `Transformer` make use of this string-indexed label, you must set the input
@@ -542,12 +542,13 @@ column, we should get the following:
 "a" gets index `0` because it is the most frequent, followed by "c" with index `1` and "b" with
 index `2`.
 
-Additionally, there are two strategies regarding how `StringIndexer` will handle
+Additionally, there are three strategies regarding how `StringIndexer` will handle
 unseen labels when you have fit a `StringIndexer` on one dataset and then use it
 to transform another:
 
 - throw an exception (which is the default)
 - skip the row containing the unseen label entirely
+- map the unseen labels with indices [numLabels]
 
 **Examples**
 
@@ -561,6 +562,7 @@ Let's go back to our previous example but this time reuse our previously defined
  1  | b
  2  | c
  3  | d
+ 4  | e
 ~~~~
 
 If you've not set how `StringIndexer` handles unseen labels or set it to
@@ -576,7 +578,22 @@ will be generated:
  2  | c        | 1.0
 ~~~~
 
-Notice that the row containing "d" does not appear.
+Notice that the rows containing "d" or "e" do not appear.
+
+If you had called `setHandleInvalid("keep")`, the following dataset
+will be generated:
+
+~~~~
+ id | category | categoryIndex
+----|----------|---------------
+ 0  | a        | 0.0
+ 1  | b        | 2.0
+ 2  | c        | 1.0
+ 3  | d        | 3.0
+ 4  | e        | 3.0
+~~~~
+
+Notice that the rows containing "d" or "e" are mapped with indices "3.0"
 
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