## What changes were proposed in this pull request?

Mark ml.classification algorithms as experimental to match Scala algorithms, update PyDoc for for thresholds on `LogisticRegression` to have same level of info as Scala, and enable mathjax for PyDoc.

## How was this patch tested?

Built docs locally & PySpark SQL tests

Author: Holden Karau <holden@us.ibm.com>

Closes #12938 from holdenk/SPARK-15162-SPARK-15164-update-some-pydocs.