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

[holdenk]

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.

[SparkQA]

Test build #57694 has finished for PR 12883 at commit 949fd16.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #57696 has finished for PR 12883 at commit 51c9ae9.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #57724 has finished for PR 12883 at commit ba4ce9d.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • class JavaClassificationModel(JavaPredictionModel):
  • class LogisticRegressionModel(JavaModel, JavaClassificationModel, JavaMLWritable, JavaMLReadable):
  • class DecisionTreeClassificationModel(DecisionTreeModel, JavaClassificationModel, JavaMLWritable,
  • class NaiveBayesModel(JavaModel, JavaClassificationModel, JavaMLWritable, JavaMLReadable):
  • class MultilayerPerceptronClassificationModel(JavaModel, JavaClassificationModel, JavaMLWritable,

[SparkQA]

Test build #57727 has finished for PR 12883 at commit 51c9ae9.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[srowen]

LGTM

[srowen]

Merged to master/2.0