Add commentary about CA deletion to plugin doc
    
    Add commentary to 'ca' plugin documentation to explain what happens
    when a CA gets deleted - namely, that its signing cert gets revoked
    and its private key deleted.
    
    Also break the docstring up into smaller chunks to aid translation.
    
    Fixes: https://fedorahosted.org/freeipa/ticket/6256
    Reviewed-By: Martin Basti <mbasti@redhat.com>