[SPARK-2841][MLlib] Documentation for feature transformations

Documentation for newly added feature transformations:
1. TF-IDF
2. StandardScaler
3. Normalizer

Author: DB Tsai <dbtsai@alpinenow.com>

Closes #2068 from dbtsai/transformer-documentation and squashes the following commits:

109f324 [DB Tsai] address feedback

(cherry picked from commit 572952ae615895efaaabcd509d582262000c0852)
Signed-off-by: Xiangrui Meng <meng@databricks.com>