This feature is mentioned in the following release notes: Development Update - August 2018, which in turn mentions bug LP: #1774206.
Looking at the MP [0], the code in the current master is the same [1]. Since it just uses plain vanilla split it seems to indicate that the list in the config should be space-separated then.
It would be good to document this for users, as well as adding it to the config description in [2].
[0] https://code.launchpad.net/~andreserl/maas/+git/maas/+merge/347398
[1] https://git.launchpad.net/maas/tree/src/maasserver/dns/config.py#n149
[2] https://git.launchpad.net/maas/tree/src/maasserver/forms/settings.py#n419