[SPARK-9797] [MLLIB] [DOC] StreamingLinearRegressionWithSGD.setConvergenceTol default value

Adds default convergence tolerance (0.001, set in `GradientDescent.convergenceTol`) to `setConvergenceTol`'s scaladoc

Author: Feynman Liang <fliang@databricks.com>

Closes #8424 from feynmanliang/SPARK-9797.

mllib/src/main/scala/org/apache/spark/mllib/regression/StreamingLinearRegressionWithSGD.scala

@@ -93,7 +93,7 @@ class StreamingLinearRegressionWithSGD private[mllib] (
   }
 
   /**
-   * Set the convergence tolerance.
+   * Set the convergence tolerance. Default: 0.001.
    */
   def setConvergenceTol(tolerance: Double): this.type = {
     this.algorithm.optimizer.setConvergenceTol(tolerance)