(doc) Document key purging in the description of the ssh_authorized_key type

This commit puts an explanation of key purging over where people will likely be
looking for it. It also tightens up the description of the user type's
purge_ssh_keys attribute, and adds an example key to the ssh_authorized_key type
(since the way that type breaks up key lines is somewhat unintuitive).

The example public key is a dud, to prevent accidental access grants.