Document default to destructive behavior

[tabowling]

@dwlehman reminder that we need to clearly communicate to end users via upstream documentation that the current behavior is to default to destructively overwriting existing and conflicting volume groups and filesystems.

We will need to file a separate issue to default to safely preserving data and having the role fail in the event of conflicts while providing the user some option to specify that they desire overwriting and destroying existing state. But for now we simply need to clearly and emphatically communicate the behavior.

[pcahyna]

I would like to know more details about the cases where the behaviour is destructive. @dwlehman can you please elaborate a bit?