[SPARK-4344][DOCS] adding documentation on spark.yarn.user.classpath.first

The documentation for the two parameters is the same with a pointer from the standalone parameter to the yarn parameter
Actually, the two config options do not work the same way. I'm working on implementing `userClassPathFirst` for yarn mode and, in the process, will deprecate `spark.yarn.user.classpath.first`, since the semantics of the former are less dangerous.

(If curious check out SPARK-2966, although the bug doesn't have a whole lot of information.)