Proposal: clarify Scala programming guide on caching ...

... with regards to saved map output. Wording taken partially from Matei Zaharia's email to the Spark user list. http://apache-spark-user-list.1001560.n3.nabble.com/performance-improvement-on-second-operation-without-caching-td5227.html

Author: Ethan Jewett <esjewett@gmail.com>

Closes #668 from esjewett/Doc-update and squashes the following commits:

11793ce [Ethan Jewett] Update based on feedback
171e670 [Ethan Jewett] Clarify Scala programming guide on caching ...