message: Add the reference of lookup.TextFileIndexer to lookup.TextFileInitializer
  based on review feedback.
diff: @@ -423,6 +423,8 @@ class KeyValueTensorInitializer(TableInitializerBase):
  class TextFileIndex(object):
  """The key and value content to get from each line.
  + This class defines the key and value used for tf.lookup.TextFileInitializer.
  +
  The key and value content to get from each line is specified either
  by the following, or a value `>=0`.
  * `TextFileIndex.LINE_NUMBER` means use the line number starting from zero,
  