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>