CDAP-1068 Correction to cdap-site.xml documentation

- Added "data.tx.timeout", default value and description; 
- corrected description of "data.tx.janitor.enable"; and
- fixed formatting of "weave.jvm.gc.opts"

**Staged at: [http://stg-web101.sw.joyent.continuuity.net/cdap](http://stg-web101.sw.joyent.continuuity.net/cdap/2.6.0-SNAPSHOT-r2.6.0_fix_config_docs/en/admin-manual/installation/cdap-site.html#appendix-cdap-site-xml)**