[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.