## What changes were proposed in this pull request?

We should now begin copying algorithm details from the spark.mllib guide to spark.ml as needed, rather than just linking back to the corresponding algorithms in the spark.mllib user guide.

## How was this patch tested?

manual review for doc.

Author: Yuhao Yang <hhbyyh@gmail.com>
Author: Yuhao Yang <yuhao.yang@intel.com>

Closes #12957 from hhbyyh/tfidfdoc.