[SPARK-4047] - Generate runtime warnings for example implementation of PageRank

Based on SPARK-2434, this PR generates runtime warnings for example implementations (Python, Scala) of PageRank.

Author: Varadharajan Mukundan <srinathsmn@gmail.com>

Closes #2894 from varadharajan/SPARK-4047 and squashes the following commits:

5f9406b [Varadharajan Mukundan] [SPARK-4047] - Point users to LogisticRegressionWithSGD and LogisticRegressionWithLBFGS instead of LogisticRegressionModel
252f595 [Varadharajan Mukundan] a. Generate runtime warnings for
05a018b [Varadharajan Mukundan] Fix PageRank implementation's package reference
5c2bf54 [Varadharajan Mukundan] [SPARK-4047] - Generate runtime warnings for example implementation of PageRank

(cherry picked from commit 974d334cf06a84317234a6c8e2e9ecca8271fa41)
Signed-off-by: Xiangrui Meng <meng@databricks.com>