Add guidelines about difference between 'oc create' and 'oc apply'

[apinnick]

Which section(s) is the issue in?

Declarative config examples

What needs fixing?

AFAIK, the imperative command 'oc create' fails if the user uses it to update a resource that already exists. Yet, I see 'oc create' used in exactly such a situation. I think it would be safer to use 'oc apply' in most cases, since it will create and update resources. I am not sure that most writers understand the difference between 'oc create' and 'oc apply'. Some guidance would help.