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
After discussion with Andreas, we have decided to remove element everywhere in the docs, and replace it with "program" (if appropriate) or "component" (if appropriate). I propose we let this go through as-is, because of all the other changes in this PR, and once this has been merged, I will create a new PR that will fix the element issue, along with some other issues.