[SPARK-15162][SPARK-15164][PYSPARK][DOCS][ML] update some pydocs

## 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.

1 files changed, 2 insertions, 3 deletions

diff --git a/mllib/src/main/scala/org/apache/spark/ml/classification/LogisticRegression.scala b/mllib/src/main/scala/org/apache/spark/ml/classification/LogisticRegression.scala
index a7ba39e432..2fa8fbcc76 100644
--- a/mllib/src/main/scala/org/apache/spark/ml/classification/LogisticRegression.scala
+++ b/mllib/src/main/scala/org/apache/spark/ml/classification/LogisticRegression.scala
@@ -72,10 +72,9 @@ private[classification] trait LogisticRegressionParams extends ProbabilisticClas
   /**
    * Get threshold for binary classification.
    *
-   * If [[threshold]] is set, returns that value.
-   * Otherwise, if [[thresholds]] is set with length 2 (i.e., binary classification),
+   * If [[thresholds]] is set with length 2 (i.e., binary classification),
    * this returns the equivalent threshold: {{{1 / (1 + thresholds(0) / thresholds(1))}}}.
-   * Otherwise, returns [[threshold]] default value.
+   * Otherwise, returns [[threshold]] if set, or its default value if unset.
    *
    * @group getParam
    * @throws IllegalArgumentException if [[thresholds]] is set to an array of length other than 2.