[SPARK-15106][PYSPARK][ML] Add PySpark package doc for ML component & remove "BETA"

## What changes were proposed in this pull request?

Copy the package documentation from Scala/Java to Python for ML package and remove beta tags. Not super sure if we want to keep the BETA tag but since we are making it the default it seems like probably the time to remove it (happy to put it back in if we want to keep it BETA).

## How was this patch tested?

Python documentation built locally as HTML and text and verified output.

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

Closes #12883 from holdenk/SPARK-15106-add-pyspark-package-doc-for-ml.

mllib/src/main/scala/org/apache/spark/ml/package-info.java

@@ -16,7 +16,7 @@
  */
 
 /**
- * Spark ML is a BETA component that adds a new set of machine learning APIs to let users quickly
+ * Spark ML is a component that adds a new set of machine learning APIs to let users quickly
  * assemble and configure practical machine learning pipelines.
  */
 @Experimental

mllib/src/main/scala/org/apache/spark/ml/package.scala

@@ -18,7 +18,7 @@
 package org.apache.spark
 
 /**
- * Spark ML is a BETA component that adds a new set of machine learning APIs to let users quickly
+ * Spark ML is a component that adds a new set of machine learning APIs to let users quickly
  * assemble and configure practical machine learning pipelines.
  *
  * @groupname param Parameters

python/pyspark/ml/__init__.py

@@ -15,6 +15,10 @@
 # limitations under the License.
 #
 
+"""
+Spark ML is a component that adds a new set of machine learning APIs to let users quickly
+assemble and configure practical machine learning pipelines.
+"""
 from pyspark.ml.base import Estimator, Model, Transformer
 from pyspark.ml.pipeline import Pipeline, PipelineModel