Fix some errors & made clarifications

This commit fixes a few errors in commit bce552063ed269490cd17c6076042a38b9ef564d, to wit:

* The Omnitruck API's options don't bear mentioning in this context since the user can't configure them anyway
* Mention that the ```cookbook_files_glob option``` is almost never used, and clarify its meaning
* Fix incorrect semantics and typos in the ```require_chef_omnibus``` option