syslog_ng: Update documentation.

Sorry for the two pull requests. After I sent the previous, I realized I should have updated the documentation as well.

The documentation reflects the function rearragement between the syslog_ng
state and execution modules.

After its merge, please take a look at it. I got rid of the print statements, and now, the execution module does the most work. The documentation is also up to date in this repo (in https://github.com/saltstack/salt, I will have to update the tutorial as well.)