[SPARK-4932] Add help comments in Analytics

Trivial modifications for usability.

Author: Takeshi Yamamuro <linguin.m.s@gmail.com>

Closes #3775 from maropu/AddHelpCommentInAnalytics and squashes the following commits:

fbea8f5 [Takeshi Yamamuro] Add help comments in Analytics

examples/src/main/scala/org/apache/spark/examples/graphx/Analytics.scala

@@ -33,6 +33,10 @@ object Analytics extends Logging {
     if (args.length < 2) {
       System.err.println(
         "Usage: Analytics <taskType> <file> --numEPart=<num_edge_partitions> [other options]")
+      System.err.println("Supported 'taskType' as follows:")
+      System.err.println("  pagerank    Compute PageRank")
+      System.err.println("  cc          Compute the connected components of vertices")
+      System.err.println("  triangles   Count the number of triangles")
       System.exit(1)
     }