doc: discuss the standard multi-tenant CephFS security model
Reviewed-by: Venky Shankar <vshankar@redhat.com>
Reviewed-by: Ramana Raja <rraja@redhat.com>
Reviewed-by: Kotresh HR <khiremat@redhat.com>
Reviewed-by: Anthony D'Atri <anthony.datri@gmail.com>
Reviewed-by: Ken Dreyer <kdreyer@redhat.com>