Added VectorIndexer section to ML user guide.  Also added javaCategoryMaps() method and Java unit test for it.

CC: mengxr

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #6255 from jkbradley/vector-indexer-guide and squashes the following commits:

dbb8c4c [Joseph K. Bradley] simplified VectorIndexerModel.javaCategoryMaps
f692084 [Joseph K. Bradley] Added VectorIndexer section to ML user guide.  Also added javaCategoryMaps() method and Java unit test for it.