Feature/r2.6.0 doc updates

Changes staged at:
http://stg-web101.sw.joyent.continuuity.net/cdap/2.6.0-SNAPSHOT/en/developers-manual/index.html

- Fixes https://issues.cask.co/browse/CDAP-728 ; see
 http://stg-web101.sw.joyent.continuuity.net/cdap/2.6.0-SNAPSHOT/en/developers-manual/building-blocks/services.html#about-path-parameters
I don't know if using element everywhere for things directly under an application is good at the moment. We have `SparkProgramController`, `FlowProgramController`, `WorkflowProgramController`, among others. Element makes sense when referring to all things of CDAP, like streams, datasets, application programs/components/things, etc., but in this case we are only referring to application programs/components/things. Since our code already refers to these application parts as "programs", I think we should leave this part as is for now.