[SPARK-7572] [MLLIB] do not import Param/Params under pyspark.ml

Remove `Param` and `Params` from `pyspark.ml` and add a section in the doc. brkyvz

Author: Xiangrui Meng <meng@databricks.com>

Closes #6094 from mengxr/SPARK-7572 and squashes the following commits:

022abd6 [Xiangrui Meng] do not import Param/Params under spark.ml

python/docs/pyspark.ml.rst

 pyspark.ml package
 =====================
 
-Module Context
+ML Pipeline APIs
 --------------
 
 .. automodule:: pyspark.ml
@@ -9,6 +9,14 @@ Module Context
     :undoc-members:
     :inherited-members:
 
+pyspark.ml.param module
+-------------------------
+
+.. automodule:: pyspark.ml.param
+    :members:
+    :undoc-members:
+    :inherited-members:
+
 pyspark.ml.feature module
 -------------------------
 

python/pyspark/ml/__init__.py

@@ -15,7 +15,6 @@
 # limitations under the License.
 #
 
-from pyspark.ml.param import *
-from pyspark.ml.pipeline import *
+from pyspark.ml.pipeline import Transformer, Estimator, Model, Pipeline, PipelineModel, Evaluator
 
-__all__ = ["Param", "Params", "Transformer", "Estimator", "Pipeline"]
+__all__ = ["Transformer", "Estimator", "Model", "Pipeline", "PipelineModel", "Evaluator"]

python/pyspark/ml/pipeline.py

@@ -22,9 +22,6 @@ from pyspark.ml.util import keyword_only
 from pyspark.mllib.common import inherit_doc
 
 
-__all__ = ['Estimator', 'Transformer', 'Pipeline', 'PipelineModel', 'Evaluator', 'Model']
-
-
 @inherit_doc
 class Estimator(Params):
     """