updates: no check for formal list validity

[sdaoden]

Describe the issue

As can be seen in the conversion of
draft-nurpmeso-smtp-tls-srv-04
the "Updates" list is not comma- but space-separated.
idnits etc did not say anything about this, even though rfcxml-vocabulary.md explicitly mentions comma-separation.

It could be this applies to other such fields, too.

Thank you!

Code of Conduct

• I agree to follow the IETF's Code of Conduct

[kesara]

Looks like this is a defect in xml2rfc.
xml2rfc does not check whether text in updates in rfc element is comma separated.
I'll transfer this issue to xml2rfc repository.