[SPARK-10670] [ML] [Doc] add api reference for ml doc

jira: https://issues.apache.org/jira/browse/SPARK-10670
In the Markdown docs for the spark.ml Programming Guide, we have code examples with codetabs for each language. We should link to each language's API docs within the corresponding codetab, but we are inconsistent about this. For an example of what we want to do, see the "Word2Vec" section in https://github.com/apache/spark/blob/64743870f23bffb8d96dcc8a0181c1452782a151/docs/ml-features.md
This JIRA is just for spark.ml, not spark.mllib

Author: Yuhao Yang <hhbyyh@gmail.com>

Closes #8901 from hhbyyh/docAPI.