[SPARK-10230] [MLLIB] Rename optimizeAlpha to optimizeDocConcentration

See [discussion](https://github.com/apache/spark/pull/8254#discussion_r37837770)

CC jkbradley

Author: Feynman Liang <fliang@databricks.com>

Closes #8422 from feynmanliang/SPARK-10230.

mllib/src/main/scala/org/apache/spark/mllib/clustering/LDAOptimizer.scala

@@ -258,7 +258,7 @@ final class OnlineLDAOptimizer extends LDAOptimizer {
   private var tau0: Double = 1024
   private var kappa: Double = 0.51
   private var miniBatchFraction: Double = 0.05
-  private var optimizeAlpha: Boolean = false
+  private var optimizeDocConcentration: Boolean = false
 
   // internal data structure
   private var docs: RDD[(Long, Vector)] = null
@@ -335,20 +335,20 @@ final class OnlineLDAOptimizer extends LDAOptimizer {
   }
 
   /**
-   * Optimize alpha, indicates whether alpha (Dirichlet parameter for document-topic distribution)
-   * will be optimized during training.
+   * Optimize docConcentration, indicates whether docConcentration (Dirichlet parameter for
+   * document-topic distribution) will be optimized during training.
    */
   @Since("1.5.0")
-  def getOptimzeAlpha: Boolean = this.optimizeAlpha
+  def getOptimizeDocConcentration: Boolean = this.optimizeDocConcentration
 
   /**
-   * Sets whether to optimize alpha parameter during training.
+   * Sets whether to optimize docConcentration parameter during training.
    *
    * Default: false
    */
   @Since("1.5.0")
-  def setOptimzeAlpha(optimizeAlpha: Boolean): this.type = {
-    this.optimizeAlpha = optimizeAlpha
+  def setOptimizeDocConcentration(optimizeDocConcentration: Boolean): this.type = {
+    this.optimizeDocConcentration = optimizeDocConcentration
     this
   }
 
@@ -458,7 +458,7 @@ final class OnlineLDAOptimizer extends LDAOptimizer {
 
     // Note that this is an optimization to avoid batch.count
     updateLambda(batchResult, (miniBatchFraction * corpusSize).ceil.toInt)
-    if (optimizeAlpha) updateAlpha(gammat)
+    if (optimizeDocConcentration) updateAlpha(gammat)
     this
   }
 

mllib/src/test/scala/org/apache/spark/mllib/clustering/LDASuite.scala

@@ -423,7 +423,7 @@ class LDASuite extends SparkFunSuite with MLlibTestSparkContext {
     val k = 2
     val docs = sc.parallelize(toyData)
     val op = new OnlineLDAOptimizer().setMiniBatchFraction(1).setTau0(1024).setKappa(0.51)
-      .setGammaShape(100).setOptimzeAlpha(true).setSampleWithReplacement(false)
+      .setGammaShape(100).setOptimizeDocConcentration(true).setSampleWithReplacement(false)
     val lda = new LDA().setK(k)
       .setDocConcentration(1D / k)
       .setTopicConcentration(0.01)