doc: explain product_id_allow_missing results in detail
    
    Add more details to the documentation for the product_id_allow_missing
    setting:
    
    - Pungi expects a product cert for each arch in each variant.
    
    - Pungi will exit with an error by default if a variant+arch's cert is
      missing.
    
    - Pungi will log a warning if product_id_allow_missing is True.
    
    Merges: https://pagure.io/pungi/pull-request/1102
    Signed-off-by: Ken Dreyer <kdreyer@redhat.com>