Clarify some issues based on disqus comment feedback.

http://docs.openstack.org/folsom/openstack-compute/install/yum/content/configure-creds.html#comment-762732821

Cherry-pick from https://review.openstack.org/19784

Change-Id: If793a0700200fa71a09500928bfdfee66375122b
Merge "Clarify some issues based on disqus comment feedback." into stable/folsom