[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
Personally, I think `spark.yarn.user.classpath.first` is a little too dangerous for us to publicly document - and we've lived so far without documenting it, and asking people to try it on a as-needed basis. But let's see if others (@tgravescs @andrewor14) have a different opinion.