[SPARK-8517][ML][DOC] Reorganizes the spark.ml user guide

docs/_data/menu-ml.yaml

@@ -1,10 +1,10 @@
-- text: Feature extraction, transformation, and selection
+- text: "Overview: estimators, transformers and pipelines"
+  url: ml-intro.html
+- text: Extracting, transforming and selecting features
   url: ml-features.html
-- text: Decision trees for classification and regression
-  url: ml-decision-tree.html
-- text: Ensembles
-  url: ml-ensembles.html
-- text: Linear methods with elastic-net regularization
-  url: ml-linear-methods.html
-- text: Multilayer perceptron classifier
-  url: ml-ann.html
+- text: Classification and Regression
+  url: ml-classification-regression.html
+- text: Clustering
+  url: ml-clustering.html
+- text: Advanced topics
+  url: ml-advanced.html

docs/ml-advanced.md

@@ -0,0 +1,13 @@
+---
+layout: global
+title: Advanced topics - spark.ml
+displayTitle: Advanced topics
+---
+
+# Optimization of linear methods
+
+The optimization algorithm underlying the implementation is called
+[Orthant-Wise Limited-memory
+QuasiNewton](http://research-srv.microsoft.com/en-us/um/people/jfgao/paper/icml07scalable.pdf)
+(OWL-QN). It is an extension of L-BFGS that can effectively handle L1
+regularization and elastic net.