add doc to LRModel's constructor

Author: mengxr

mllib/src/main/scala/org/apache/spark/mllib/classification/LogisticRegression.scala

@@ -44,6 +44,9 @@ class LogisticRegressionModel (
     val numClasses: Int)
   extends GeneralizedLinearModel(weights, intercept) with ClassificationModel with Serializable {
 
+  /**
+   * Constructs a [[LogisticRegressionModel]] with weights and intercept for binary classification.
+   */
   def this(weights: Vector, intercept: Double) = this(weights, intercept, weights.size, 2)
 
   private var threshold: Option[Double] = Some(0.5)