Fixes #7091/BZ1129375: fix documentation for create/update system.

The params documentation for create and update system was incorrect.
This commit fixes the documentation.

http://projects.theforeman.org/issues/7091
https://bugzilla.redhat.com/show_bug.cgi?id=1129375
Merge pull request #4594 from waldenraines/7091

Fixes #7091/BZ1129375: fix documentation for create/update system.