(CDAP-703) Doc Changes due to unification of APIs: MapReduce and Service changes

Partial fixes for https://issues.cask.co/browse/CDAP-703 and https://issues.cask.co/browse/CDAP-460. 
(Remainder of work will have to wait until 2.7.0.)

Staged at:
- http://stg-web101.sw.joyent.continuuity.net/cdap/2.6.0-SNAPSHOT-2.6.0_doc_updates/en/developers-manual/building-blocks/mapreduce-jobs.html
- http://stg-web101.sw.joyent.continuuity.net/cdap/2.6.0-SNAPSHOT-2.6.0_doc_updates/en/developers-manual/building-blocks/services.html