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>