[SPARK-8949] Print warnings when using preferred locations feature

Add warnings according to SPARK-8949 in `SparkContext`
  - warnings in scaladoc
  - log warnings when preferred locations feature is used through `SparkContext`'s constructor

However I didn't found any documentation reference of this feature. Please direct me if you know any reference to this feature.

Author: Han JU <ju.han.felix@gmail.com>

Closes #7874 from darkjh/SPARK-8949.

core/src/main/scala/org/apache/spark/SparkContext.scala

@@ -118,9 +118,11 @@ class SparkContext(config: SparkConf) extends Logging with ExecutorAllocationCli
    * Can be generated using [[org.apache.spark.scheduler.InputFormatInfo.computePreferredLocations]]
    * from a list of input files or InputFormats for the application.
    */
+  @deprecated("Passing in preferred locations has no effect at all, see SPARK-8949", "1.5.0")
   @DeveloperApi
   def this(config: SparkConf, preferredNodeLocationData: Map[String, Set[SplitInfo]]) = {
     this(config)
+    logWarning("Passing in preferred locations has no effect at all, see SPARK-8949")
     this.preferredNodeLocationData = preferredNodeLocationData
   }
 
@@ -153,6 +155,9 @@ class SparkContext(config: SparkConf) extends Logging with ExecutorAllocationCli
       preferredNodeLocationData: Map[String, Set[SplitInfo]] = Map()) =
   {
     this(SparkContext.updatedConf(new SparkConf(), master, appName, sparkHome, jars, environment))
+    if (preferredNodeLocationData.nonEmpty) {
+      logWarning("Passing in preferred locations has no effect at all, see SPARK-8949")
+    }
     this.preferredNodeLocationData = preferredNodeLocationData
   }