update comments

Author: hhbyyh

mllib/src/main/scala/org/apache/spark/mllib/linalg/Matrices.scala

@@ -940,11 +940,14 @@ object Matrices {
       case dm: BDM[Double] =>
         new DenseMatrix(dm.rows, dm.cols, dm.data, dm.isTranspose)
       case sm: BSM[Double] =>
-        val mat = if(sm.colPtrs.last != sm.data.length) {
+        // Spark-11507. work around breeze issue 479.
+        val mat = if (sm.colPtrs.last != sm.data.length) {
           val matCopy = sm.copy
           matCopy.compact()
           matCopy
-        } else sm
+        } else {
+          sm
+        }
         // There is no isTranspose flag for sparse matrices in Breeze
         new SparseMatrix(mat.rows, mat.cols, mat.colPtrs, mat.rowIndices, mat.data)
       case _ =>

mllib/src/test/scala/org/apache/spark/mllib/linalg/MatricesSuite.scala

@@ -498,7 +498,7 @@ class MatricesSuite extends SparkFunSuite {
 
   test("fromBreeze with sparse matrix") {
     // colPtr.last does NOT always equal to values.length in breeze SCSMatrix and
-    // invocation of compact() is necessary. Refer to SPARK-11507
+    // invocation of compact() may be necessary. Refer to SPARK-11507
     val bm1: BM[Double] = new CSCMatrix[Double](
       Array(1.0, 1, 1), 3, 3, Array(0, 1, 2, 3), Array(0, 1, 2))
     val bm2: BM[Double] = new CSCMatrix[Double](