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 agree. Also, "Element" was a term that is associated with a Reactor. We should not change anything to "element"