[SPARK-2842][MLlib]Word2Vec documentation

mengxr
Documentation for Word2Vec

Author: Liquan Pei <liquanpei@gmail.com>

Closes #2003 from Ishiihara/Word2Vec-doc and squashes the following commits:

4ff11d4 [Liquan Pei] minor fix
8d7458f [Liquan Pei] code reformat
6df0dcb [Liquan Pei] add Word2Vec documentation

(cherry picked from commit eef779b8d631de971d440051cae21040f4de558f)
Signed-off-by: Xiangrui Meng <meng@databricks.com>