Change docs on 'reduce' since the merging of local reduces no longer preserves
ordering, so the reduce function must also be commutative.
Merge branch 'master' of https://github.com/mesos/spark into commutative

Conflicts:
	core/src/main/scala/spark/RDD.scala
Merge pull request #453 from markhamstra/commutative

Change docs on 'reduce' since the merging of local reduces no longer pre...